UX: allow to specify -H with IP address or FQDN

previously if an hypervisor was added to the engine
using its FQDN specifying its IP address to -H (or
vice versa) caused an error like:
'The following host are not listed as hypervisors:...'

now provided that FQDN resolve to IP and IP reverse
resolve to FQDN you can specify indifferently one
of them

Bug-Url: https://bugzilla.redhat.com/950336
Change-Id: I7b63184de64fe64f1ebfd925e9f787416d8e3d8c
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>