Multi instance launch

[zhxu73]

Description

Enable launching multiple instances with a single request.
Multi-instance launch request is just the normal launch request with an extra parameter.
e.g.

{
"extra": {"instance_count": 2}
}

Checklist before merging Pull Requests

• New test(s) included to reproduce the bug/verify the feature
• Add an entry in the changelog
• Documentation created/updated (include links)
• If creating/modifying DB models which will contain secrets or sensitive information, PR to clank updating sanitation queries in roles/sanitary-sql-access/templates/sanitize-dump.sh.j2
• Reviewed and approved by at least one other contributor.
• New variables supported in Clank

[coveralls]

Coverage decreased (-0.07%) to 39.143% when pulling 9907f4f on zhxu73:multi-instance-launch into ebe5aa7 on cyverse:master.