UI: Cloud-Init gateway is not mandatory

Remove the force validation for gateway when choosing Clout-Init at
run-once.

Change-Id: I9b8fb3fd84158ee7c80662e85125b26d22148daa
Bug-Url: https://bugzilla.redhat.com/1047345
Signed-off-by: Shahar Havivi <shaharh@redhat.com>