Add --plugin-download-directory option to examples

[aSky17]

fixes #607
Added --download-directory-option in README.md along with added the path where to install a plugin

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[timja]

I think ok, not 100% sure on the docker one it might confuse people as I think it defaults to the right place there, @MarkEWaite ?

[MarkEWaite]

Thanks for the pull request @aSky17 . I think that we should use a consistent path to the plugins directory in the two examples. I've proposed changes to make the new syntax work that way.

I think that the container example needs a different approach. I've suggested that we leave it unchanged in this pull request and that a later pull request be used to update the container based example.

Later pull requests might make the following additional changes:

• Use /your/path/to/ as the directory prefix in both examples (one pull request)
• Adapt the container example to actually update the plugins in a running container (another pull request, and likely much more complicated)

The container example pull request needs more steps than are currently listed. I was able to make the container example work with the following steps:

docker cp /your/path/to/plugins.txt <container_name>:/tmp/plugins.txt
docker exec -it <container_name> /bin/bash
jenkins-plugin-cli --plugin-file /tmp/plugins.txt
cp -r -p /usr/share/jenkins/ref/plugins/. /var/jenkins_home/plugins/.
exit

After performing those steps, the newly downloaded plugins were visible in the container image that I was running. I think those steps are complicated enough that they deserve a separate pull request.

[MarkEWaite]

I think that we should use a consistent prefix string and use the name plugins rather than plugin since the directory plugins is where Jenkins looks for its plugins. I would prefer to include the trailing '/' character on the directory path in hopes of showing the user that the argument is a directory name and not a file name.

Suggested change

	java -jar jenkins-plugin-manager-*.jar --war /your/path/to/jenkins.war --plugin-download-directory /path/where/to/download/plugin --plugin-file /your/path/to/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin
	java -jar jenkins-plugin-manager-*.jar --war /your/path/to/jenkins.war --plugin-download-directory /your/path/to/plugins/ --plugin-file /your/path/to/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin

[MarkEWaite]

I think that we should use the same form of plugins directory path in both examples. I think that once this pull request is merged we should submit a new pull request that makes the directory paths in this example consistent with the directory paths in the previous example. No need to disrupt this pull request with that change, but it would be worth doing in a later pull request.

Suggested change

	java -jar plugin-management-cli/target/jenkins-plugin-manager-*.jar --war /file/path/jenkins.war --plugin-download-directory /path/where/to/download/plugin --plugin-file /file/path/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin
	java -jar plugin-management-cli/target/jenkins-plugin-manager-*.jar --war /file/path/jenkins.war --plugin-download-directory /your/path/to/plugins/ --plugin-file /file/path/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin

[MarkEWaite]

I think that @timja is correct that we should not adjust the download directory in this example because it is inside the Jenkins container image and that container image has an existing directory structure that we should use. The existing example does not use the existing directory and file conventions that are used in the container images. I don't thin we should change it in this pull request. Let's save that for a future pull request

Suggested change

	jenkins-plugin-cli --plugin-download-directory /path/where/to/download/plugin --plugin-file /your/path/to/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin
	jenkins-plugin-cli --plugin-file /your/path/to/plugins.txt --plugins delivery-pipeline-plugin:1.3.2 deployit-plugin

[MarkEWaite]

Thanks!