Using the Trove devstack plugin was dependent on having the
python-troveclient also enabled. This change modifies the plugin.sh
file to install the client correctly, thus removing the need of
separately enabling a plugin for it.
Variables were set up in the devstack settings file, which can be
overridden as desired.
A README.rst file was added with instructions on how to set up the
plugin in devstack.
Change-Id: If5a76360249827c19759a5c90dc00f70c59c32cc