MaaS: preseed Armband repo via curtin

MaaS: preseed Armband repo via curtin

Instead of installing upstream vanilla kernel from Ubuntu just to
upgrade to the one from Armband repo later, preseed Armband repo
config via curtin.
This allows us to drop a kernel update cycle, thus saving a few
minutes at each deploy.

While at it, switch salt repo addition to use the maas region reclass
section instead of adding a dedicated mechanism for it in the curtin
template (only for arm64).

JIRA: ARMBAND-280

Change-Id: Iad5953a7de80d728f11a4ce3113bd7725100c31c
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>