reclass: Multiarch support for arch specific param

reclass: Multiarch support for arch specific param

Introduce a simple mechanism that simulates an 'if-arch-then' cond
for reclass models:
- add new <all-mcp-ocata-common> class hierarchy;
- at runtime (via <salt.sh>) make 'all-mcp-ocata-common.arch' point
  to 'all-mcp-ocata-common.$(uname -i)' dynamically;
- inherit new 'arch' class in all cluster models;
- factor out current x86_64 default for "salt_control_xenial_image";
- add AArch64 default for param "salt_control_xenial_image";

Change-Id: I3b239b28d0fd1cc2ced8579e2e93b764eb71ffc3
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>