Add ContentActionClient

Replace RepoActionInvoker in main ActionClient with a
ContentActionClient.

Split BaseActionClient to its own module.
Add ostree_action_invoker (ostree repo action)

Hook it up into ContentActionClient
Add ContentActionPluginConduit plugin.

This conduit is for plugins that will add
content actions used in ContentActionClient.

Rev plugin api version to '1.1' since this is
a backwards compatible change. And I remembered.
Add a dummy yum_content plugin for testing
Load content actions from plugin class search

The content action client, now uses the subscription
manager plugin hook 'content_plugin_search' to find
ContentActionInvoker style classes, construct them,
and add them to the list that ContentActionClient
will run.
Replace odd usage of 'repolib' name.

This code actually passes a RepoActionInvoker now,
so remove the confusing repolib name.
entcertlib.repo_hook can be content type agnostic

More than likely, we can remove this tightly coupled
if we make sure entcert updates are via an ActionClient
that will follow up and update content.
Add test for ContentActionPluginConduit
Hit ContentActionClient for some test coverage.

It's tests need to get split out at some point.
Remove sample yum_content plugin

It will re emerge as ostree plugin in
src/content_plugins/ostree
Add content_plugins dir,and ostree content plugins
HACK: install ostree content plugin in site-packages

/usr/share/rhsm-plugins/ is not on sys.path, so without
append to path, we can't include submodules there.

Plugins can install their helper modules to standard
python paths. At the moment, hardcode
/usr/lib/python2.7/site-packages/rhsm_content_plugins

Need to generalize that. Plugins may need their
own install process. But this get's things testable.
Add ostree_content plugin entry point

It's modules can live in site-packages
Log exceptions from trying to load plugins.
Make ostree content action loaded from plugins.
Remove unneeded #!/usr/bin/python

This module is not invoked as a main, only
imported.
Move OstreeContentActionInvoker to it's own module
Add TODO about content_plugin installs

To install content plugins:
   make install-content-plugins
   make install-content-plugins-conf

src/content_plugins/ostree_content.py is the ostree_content
plugin entry point.

ostree_content specific modules can live in
src/content_plugins/ostree/. That dir gets installed to
/usr/lib/python2.7/site-packages/rhsm_content_plugins/ostree/

(yes, hardcoded atm)
Start adding ostree implementation
Start ostree implementation tests