Increase testing of network connectivity

Adds ping against VM's internal network elements:
need all of them because agents might be located on different hosts
    1. Router port
    2. DHCP port

Adds ping against public network default gateway implying external
connectivity. Any further network tests are outside the Openstack cloud and the
testing environment

Partially Implements: blueprint neutron-advanced-scenarios

Change-Id: I9aaba5496581c9e3472d7a3846b1a0464f6334e6
Merge "Increase testing of network connectivity"