Implement trove client plugin

This moves the client creation code out of Clients._trove() into
its own client plugin.

Trove is an integrated project, and python-troveclient is a dependency,
so the import is now mandatory.

Change-Id: I89d290f6a38488779e77ff11bdfdd64c159a57e8
Implement ceilometer client plugin

This moves the client creation code out of Clients._ceilometer() into
its own client plugin.

Ceilometer is an integrated project, and python-ceilometerclient
is a dependency, so the import is now mandatory.

Change-Id: Id4961c4d9678d7bfe3739128843c41921cc8a34b
Implement heat client plugin

This moves the client creation code out of Clients._heat() into
its own client plugin.

Clients._get_client_option is no longer used, so has been deleted

Change-Id: Ie14906b6cc605d6db49a5153ffa27f82debcc430
Merge "Implement trove client plugin"
Merge "Implement ceilometer client plugin"
Merge "Implement heat client plugin"