Make domain_id immutable by default

We already allow the domain_id in User, Group and Project entities
to be made immutable by use of a config option.  By default, however,
the domain_id is mutable.

This patch switches this so that the domain_id is immutable by default.
Although this changes existing functionality, it is felt that since
nearly all non-trivial production implementations that use domains
are going to want to restrict domain admin personas with a suitable
policy file, leaving the domain_id mutable by default represents a
potential vunerability that could be exploited.

Closes-Bug: 1294293

Change-Id: I0cb9fd4dc520d0a15bf54f4d138a3794d0ccd1d9
Merge "Make domain_id immutable by default"