Add some notes on trove image
Add some notes on trove image

The trove image is one of the larger ones we cache, and after
double-checking it is still used as part of the
gate-tempest-dsvm-trove tests (although integration tests seem use
their own dib-built version)

Add some notes to clarify the situation with this image.

Change-Id: I2319dd5811d6bd215d1e8778eca5b4c9399f0efb

openstack-dev/devstack,bswartz/devstack,group-policy/devstack,richliu/devstack,samgoon/devstack,nawawi/openstack,bigswitch/devstack,bswartz/devstack,bigswitch/devstack,nawawi/openstack,richliu/devstack,dtroyer/devstack,Mahito/devstack,openstack-dev/devstack,samgoon/devstack,dtroyer/devstack,Mahito/devstack,noironetworks/devstack,group-policy/devstack,noironetworks/devstack