Sphinx-ify QuantumPluginBaseV2 docstrings

Sphinx-ify QuantumPluginBaseV2 docstrings

The quantum.quantum_plugin_base_v2 module had some good docstrings,
but there was no corresponding sphinx code that would generate
docs from them. There were also some syntax issues with the
sphinx markup in the docstrings.

This change adds sphinx directives so these docs will be auto-
generated. It also modifies the docstrings in the QuantumPluginBaseV2
methods so they are parsed by the Sphinx documentation generator
without any errors.

If this patch is accepted, a "Plugin API" link to these docs
will appear at

http://docs.openstack.org/developer/quantum/

Fixes bug 1186255

Change-Id: I96eddcc516d109766d7f4c52edd595748696b595