Convert to using newly imported processutils.

This change moves nova over to using the newly imported trycmd() and
ssh_execute() implementations from oslo's processutils.

Change-Id: Ied2bb1eeb0cbdc6903a4028a60c89ca9e23656b4
Show the cause of virt driver error

Make sure that the virt driver loading errors are shown with the
stacktrace attached. Otherwise the source of an import error can be
completely hidden. This is important here, since virt driver can fail
due to various issues, like quantum vif problem.

Change-Id: Icb90d348e4a4ddef5768ca32836bd95d94981499
Merge "Convert to using newly imported processutils."
Merge "Show the cause of virt driver error"