Replace custom cinder driver configs

The devstack/lib/cinder file has a number of third party
driver config options hard-coded in it.  Rather than add
yet another if driver== statement here let's use plugin
files and do something similar to what's already
in place for nova_hypervisors and neutron plugins.

This works the same way folks were implementing their
drivers already, the key is to use a CINDER_DRIVER
variable in your localrc file that matches the name
of the lib/cinder_plugin file to use.

The existing third party driver entries that were
in lib/cinder have been migrated to cooresponding
plugin files.

Change-Id: I4ee51ea542d5aa63879afd5297311a9df727c57f
Merge "Replace custom cinder driver configs"