Add test to find out statistic of connectivity failures
in case of neutron agents migration

- Add test in which we manually migrate l3 and dhcp agent
and then check instance connectivity
- Add methods to work with neutron client to OpenstackActions class

Change-Id: I7645962889d729dca84a72ae725b3c281ce497b2
Closes-Bug: #1396522
Merge "Add test to find out statistic of connectivity failures in case of neutron agents migration" into stable/5.1