Allow specifying the gateway_external_network by name

This change makes the l3_agent recipe lookup the ID of the network used
for the gateway external network by name.  For example, specify

node[openstack][network][l3][gateway_external_network_name] = public

and the recipe will lookup the network by name.  It will use
openstack-common/libraries/cli.rb network_uuid method, effectively
querying `neutron net-external-list`.  The result is saved to
gateway_external_network_id and used on subsequent runs.

To have the recipe check the UUID again (e.g. to update it), reset the
_id attribute to nil.

Change-Id: I5df6f9989df7b18121a8050835a192da9e1a762e
Merge "Allow specifying the gateway_external_network by name"