New TEMPEST_VOLUME_REVERT_TO_SNAPSHOT setting
New TEMPEST_VOLUME_REVERT_TO_SNAPSHOT setting

Follow the pattern of the other configuration keys. The new variables
allows tests to enable/disable volume revert tests provided
by cinder-tempest-plugin.

Revert-to-snapshot was introduced in pike, and so the tests.

Change-Id: If137f201c2f646703f5a1ff96e71e48caed63b67

nawawi/openstack,openstack-dev/devstack,openstack-dev/devstack,nawawi/openstack