core: VdsProperties - rename NETWORK_INTERFACE

VdsProperties -> NETWORK_INTERFACE constant is used both for
network-interface and disk-interface.
Hence, renaming to the generic concept: INTERFACE.

Change-Id: I208f8369084d5a3d42cfb7ebb592569f6063a4d3
Signed-off-by: Daniel Erez <derez@redhat.com>