[cern] Add master_availability_zones label
master_availability_zones is a comma_delimited_list of availability_zones. master-0 gets az master_availability_zones[0], master-N gets az master_availability_zones[N].
If the labels availability_zone is set all nodes (master and worker) will use this availability zone, for backwards compatibility.
closes #1 (closed)
Change-Id: I5c7c0b0819fe350dce58ab8175466eda9e285c8e Signed-off-by: Spyros Trigazis spyridon.trigazis@cern.ch