Add more neutron tests and supporting methods

Add test test_ports_extra_dhcp_options()
Add test test_enable_dhcp()
Add test test_ip_allocation_order()
Correct a log statement position
add option to include dhcp option in create_port()
add update_port() method
add methods wait_till_vm_boots() and get_console_output() in VM fixture
Fix bug in test_dns_nameservers()
Enhance test_allocation_pools()

Change-Id: I6679778f156ad8dd3aca6cbf139394ee70edd749