Fix ubuntu and centos static_net cobbler snippet

Previously, we identified admin net by
interface name, but bootstrap interface
ordering differs from ubuntu's, so it is
necessary to enumerate manually and use
mac address to identify which interface
is on the admin network.

CentOS snippet also updated to reflect changes
to choose_interface providing MAC instead of
interface name

Change-Id: I01f2df69fd98e614129368cea605d466d604d2a0
Closes-Bug: #1269920