Fix ssh connection recreation in generic driver

When ssh connection died, generic driver tries remove broken connection by
passing nonexistent value, that is never assigned. Set expected value for
removal. Also added unittest for '_ssh_exec' method of generic driver due to
absense of it.

Change-Id: I7820a97d0aff52163b3ae56c9441323d7883f971
Closes-Bug: #1364848