make tg_group.group_name not NULLable

Having NULL for the group_name doesn't make sense, and will screw up
lots of code (including the new LDAP support).

Change-Id: I0cf560ee10cb2dc0e0e3ee4bbace15e3c47227e8
Merge "make tg_group.group_name not NULLable" into develop