core: 'Start-on-boot' of a nic should be respected

The user should be able to instruct which network interface
will be booted on startup when the interface is configured
by cloud-init.

Change-Id: I325c5edaa27be322bd5488d4397f696338e434fe
Bug-Url: https://bugzilla.redhat.com/1132439
Signed-off-by: Moti Asayag <masayag@redhat.com>