openstack case fail when there is a public network with rbac for specified tenant

[Yang Li]

The current logic to set public network is get the first public network from neutronclien,
but this network could only be seen for one tenant if other users have set rbac policy, if
this network was written into configuration, some case will fail for:
Network xxxxxxxxxxx could not be found
or
Floating IP pool not found.

So we should get the public network which can be seen by all tenants.
Also if there is no rbac policy, we still use the first public network.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/626426