Handle non-ascii characters in spawn exception msg

If a compute driver's spawn function returns an exception whose text
has any non-ascii characters (e.g., for globalization), the compute
manager's "reschedule logic" doesn't handle it properly as it runs
the text through the str() function, which causes UnicodeEncodeError
exceptions.

This simply sends the exception text through six.text_type so as to
no longer blow up and proceed with rescheduling as intended.

Closes-Bug: #1359427

Change-Id: I3fef783feffbe99c6971543c881e3fce2b60693b
Merge "Handle non-ascii characters in spawn exception msg"