Support client certificates for HTTP requests

[sykaeh]

This fixes #737

I also used this opportunity to consolidate the parameters that are supplied for HTTP requests and added the parameter parsing in the general CollectorBot.

There also seemed to be inconsistencies whether http_ssl_proxy or https_proxy should be used. I replaced all of them with https_proxy (seemed to be more common and documented).

[sebix]

• Renaming parameters means breaking installations
• The newly added parameters are not documented in docs/Bots.md
• http_proxy and https_proxy are provided by defaults as None, you explicitly change it to empty string :(
• As they are always provided, no need to use the getattr workaround
• ssl_cl_cert is not self-explaining, better use ssl_client_cert

[aaronkaplan]

overall : looks ok. Did a brief review of the code. Please let's test this internally and merge.
Thanks @sykaeh

[codecov-io]

Current coverage is 75.70% (diff: 5.26%)

Merging #767 into master will increase coverage by 0.16%

@@             master       #767   diff @@
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  Files           223        223          
  Lines          8167       8151    -16   
  Methods           0          0          
  Messages          0          0          
  Branches          0          0          
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+ Hits           6170       6171     +1   
+ Misses         1997       1980    -17   
  Partials          0          0          

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[sykaeh]

Renaming parameters means breaking installations

I think for the sake of simplicity and clarity it is worth it. Also, if we change it before 1.0, we do not have to try to maintain the inconsistencies in the future.

The newly added parameters are not documented in docs/Bots.md

There are a lot of things that are not documented in docs/Bots.md. I have now added all of the generic collectors along with their parameters. As I have mentioned before in #757, it would be nice to have a better solution to this. Also, I do not understand what exactly is meant by name, lookup and public in the Bots.md file.

http_proxy and https_proxy are provided by defaults as None, you explicitly change it to empty string :(

Sorry, I hadn't noticed that. I changed it to null.

As they are always provided, no need to use the getattr workaround

I just copied what was done before, but I agree and have now changed it.

ssl_cl_cert is not self-explaining, better use ssl_client_cert

Same, I just copied what was done before, but I changed it to ssl_client_cert