VirtualPowerDriver catches ProcessExecutionError.

Baremetal virtual_power_driver now catches ProcessExecutionError coming
from ssh_execute and logs an exception. Requested actions (eg, power_on)
return an error state instead of raising an exception.

Change-Id: I8f66ba843e12de13f9a14d736f7bef2eda4ef85c
Merge "VirtualPowerDriver catches ProcessExecutionError."