Correct SSHPowerDriver validate() exceptions

Driver valdiate() method should raise only InvalidParameterValue
exceptions, which will be caught by
ConductorManager.validate_driver_interfaces().

The SSH driver was potentially raising a FileNotFound exception which
would be unhandled. This patch converts that into InvalidParamterValue.

Change-Id: I82d9776206c1391ba151614ac7512865f8ea672c
Merge "Correct SSHPowerDriver validate() exceptions"