salt.sh, user-data: Add Saltstack arm64 repo

salt.sh, user-data: Add Saltstack arm64 repo

These changes will only be applied on AArch64, based on `uname -m`.

While at it, add arm64 suppport to salt-master-setup.sh.
Upstream commit [1] broke Salt bootstrap on AArch64, by
introducing an architecture condition that is too strict to allow
Debian package installation (even if we provide our own repo).

Add "arm64" to the list of supported architectures. This needs
to be done on the fly, as the bootstrap script is fetched using
`curl` from <salt-master-setup.sh>.

[1] https://github.com/saltstack/salt-bootstrap/commit/caa6d7d

Change-Id: Id706a74a52ffe2f8b8c9dd3b9f70c78f35b2f745
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>