core: fixing nfs mountpoint FQDN regex

*the regex now supports fqdn similar to: "foo.example.f7:/<export>"
*minor structural fixes of the regex

Change-Id: Iba3121b602d9e2e21bc120875cdfc4298f872841
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1126342
Signed-off-by: Liron Aravot <laravot@redhat.com>