[baremetal] Fix cmp proto dhcp on PXE/admin

[baremetal] Fix cmp proto dhcp on PXE/admin

Although we properly filter the PXE/admin interface in the common
openstack_compute_pdf.yml.j2 template and use DHCP instead of manual
setup, we failed to do the same in scenario-specific overrides
(ODL, OVS), so we end up with 'proto: manual' on PXE/admin on cmp
nodes.

The fix is trivial and reuses the mechanism in the common class in
scenario-specific templates (if interface is PXE/admin, use 'DHCP'
instead of 'manual').

This solves the issue of broken connectivity to Salt master after
cmp reboot.

Change-Id: I1953d03343190acb2efcab4412a3d37e130b0ea9
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>