Adding  $deployable_networks variable in network.pp

This refernces the already defined networks inside of the
$all_network_subnets variable, rather than copy/pasting
them again.

This does not remove $mw_appserver_networks, as it is being
used in other places that should be limited to only
mediawiki related deploys.

See also: I2891dcb8ef51706ca20770253eb7ea1eebea0cba

Change-Id: I0a175b4a23002eac275fd6172da8d13be9acff2c