rhel: s/OVN_DB/OVS_DB/ in ovn-controller unit.

rhel: s/OVN_DB/OVS_DB/ in ovn-controller unit.

I added a variable called OVN_DB, but had mixed up what this parameter
to ovn-controller was for.  This parameter is the location of the db
for the local ovs-vswitchd.  It then gets the OVN database location
from *that* db.  It seems fine to keep the env var in case someone
needs to override it for some reason, but correct the name and
description of what it is.

Signed-off-by: Russell Bryant <rbryant@redhat.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>