engine: boot-protocol for VmInitNetwork

Needs to return STATIC instead of STATIC_IP
Validate BootProtocol input.

Bug-Url: https://bugzilla.redhat.com/1144005
Change-Id: I21f8a9cf509dd4d3d6e73a83ce9f66b183605161
Signed-off-by: Shahar Havivi <shaharh@redhat.com>