netcmd: models: gmsa move GroupManagedServiceAccount model to gmsa.py

netcmd: models: gmsa move GroupManagedServiceAccount model to gmsa.py

It needs to inherit from the Computer model, the Computer model also inherits from User.

First, moving it to its own file from user.py to gmsa.py

Signed-off-by: Rob van der Linde <rob@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>