neutron's test test_create_floatingip_with_specified_ip_address should be executed only for ML2 plugin

test neutron.tests.tempest.api.admin.test_floating_ips_admin_actions.FloatingIPAdminTestJSON.test_create_floatingip_with_specified_ip_address uses specific feature of ML2 plugin - it tries to detect allocated floating ips through listing of ports.
This way doesn't work for non ML2 plugins (ex: Contrail).

Contrail plugin doesn't create port for newly allocated floating ip like ML2. When common test's function get_unused_ip returns floating ip that is allocated but not attached then then test case tries to allocate it again it gets HTTP 409.

It can be one of these changes to fix this:
1. Do not create a floating ip in setup (it wouldn't help if IP was allocated by someone else)
2. When finding first free ip, also skip over floating ip objects.
3. Use a random available ip address for the test.