Code refactoring and adding ssh test

Classes L2HostroutesGatewayMixin and Instance are refactored to a common module
so other scenario tests can use them in the future.

ssh connectivity test between 'origin' and 'destination' instances is added to
L2HostroutesGatewayTest.

Change-Id: Ic99c8f06930c7b13fd94f12be12ab0367488b23d
Merge "Code refactoring and adding ssh test"