We want to run ansible tests against some public cloud, but
there are always some API policy limits for regular users in
public cloud, like: can not create external network. So we define
some variables that can be changed for different limit scenairoes.
1. Create port in external or internal network.
2. Specified public pool name when launching server.
3. Create simple router, then add interface and external gateway,
change test step order, so that do not create external network
when network_external=false.
4. Create subnet with enable_dhcp true or false.
5. Make ANSIBLE_VAR_* environment variables can be passed into virtual
env.
Change-Id: I69473756b23a6cb525e0f9bb40d09e6ed9880782
722d8136b2