engine: refactoring NetworkUtils.getVlanDeviceName

This patch changes the method name to constructVlanDeviceName.
The method should be used only for constructing new vlan devices.
Other places shoulded assume the vlan device name is in format-
baseIface.vlanId

Change-Id: I34ed8b14fb8e22d9681bdbc34d863c52b6d23be5
Bug-Url: https://bugzilla.redhat.com/1091863
Bug-Url: https://bugzilla.redhat.com/999975
Signed-off-by: Alona Kaplan <alkaplan@redhat.com>