Remove Prefix From Datastore Version Name

given the agreement to concatenate/consolidate datastore-type and
datastore-version for Horizon, it makes sense to change the default
datastore-version names for devstack/redstack, otherwise it will
appear as "mysql-mysql-5.5" vs. something more sensical like
"mysql-5.5".

Change-Id: I2c15ad0860c0c9541390bed891cc5e7ba461e6de
Closes-Bug: #1308809