Assign floating ip to a server in load balancer scenario

In isolated jobs tenant network is being created for each test
and host doesn't have direct access to it. To solve this problem
we need to assign a floating ip to the server and use it instead.

A floating ip is assigned only in case public_network_id is defined
and tenant_networks_reachable is False. If tenant_networks_reachable
is True use direct addressing.

Change-Id: I3943e57c040f4a5f7c36d42fa100bbc324097159
Closes-Bug: #1277381
Merge "Assign floating ip to a server in load balancer scenario"