[maas] AArch64: Preseed Armband repositories

[maas] AArch64: Preseed Armband repositories

Preseed Armband common repo-comp for Pike, so we get
the updated kernel & other packages from the start.
Requires upstream PR [1].

While at it, also handle related PR merged upstream [2] by
explicitly setting maas.region.subnets name via j2 instead of reclass
param expansion in name (allows us to drop the remaining chunk of the
fabric-from-CIDR patch we used to carry).

Note: opnfv_infra_maas_pxe_network_addres can now be dropped from
pod_config j2 template in Pharos Fuel installer adapter.

[1] https://github.com/salt-formulas/salt-formula-maas/pull/26
[2] https://github.com/salt-formulas/salt-formula-maas/pull/22

Change-Id: I356adb73b80f0f2d85db4ab060e804cb9a053862
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>