tools: Allow to configure domains_realm section via external file

This can be handy in case the kerberos realm for the
added domain is not an upper case version of it.
Bare in mind the external file should contain all the domains
in case there is more than one domain in the setup

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1070465
Change-Id: I970b8f4950abb5eeb0a72a46ed00af26028919dd
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>