Rbarlow ssl verify option

This pull request addresses these three BZs:

https://bugzilla.redhat.com/show_bug.cgi?id=1112904
https://bugzilla.redhat.com/show_bug.cgi?id=1112906
https://bugzilla.redhat.com/show_bug.cgi?id=1113590

For the purposes of speed, this pull request does not deal with documentation of these changes, and does not include any new tests to cover the new features and settings introduced. Another pull request will be submitted soon with these changes.

This pull request also does not allow disabling CA signature checking for the agent's pulpplugin.py module. It was unexpectedly complicated, and it was believed to be less important than these other changes.
Ah yes, 2.4 ☹