core: unlock_entity.sh fails on authentication

The DB was configured in pg_hba.conf with md5
This means that in order to run this utility properly you should set the
PGPASSWORD or  PGPASSFILE environment variable.

Example:

 \$ PGPASSWORD=xxxxxx ./unlock_entity.sh -t disk -q

I had added this information to the utility help

Change-Id: I6055bec6b91d5eb101e6655ed4eea481a6a082f9
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1118191
Signed-off-by: Eli Mesika <emesika@redhat.com>