[salt] Retire Armband Saltstack repos

[salt] Retire Armband Saltstack repos

Provided we take care of the minor consequences of relying strictly
on upstream Saltstack amd64 repos on arm64 (since we don't use RAET,
this means merely installing `python-futures` as a dependency of
`python-tornado` on all minions), we can finally drop our Salstack
arm64 repository.

This implies installing python-futures on:
- FN and VCP VMs (handled via lib.sh pre-patching);
- baremetal nodes (handled via MaaS Curtin template for arm64 now);

While at it, also handle other minor patch cleanup operations:
 * drop `system.linux.system.repo.mcp.armband`, already covered by:
   - lib.sh pre-patching for FN and VCP VM images;
   - maas.region.repositories for baremetal nodes;
 * switch glusterfs PPA repo arch from 'amd64,arm64' to None;
 * drop obsolete patch handling `mcelog` on AArch64;

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

JIRA: ARMBAND-382

Change-Id: I34f3bd70ebf10395e8d4e70498ff3a144b4d9c3b
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>