Mask passwords in exceptions and error messages

When a ProcessExecutionError is thrown by processutils.ssh_execute(),
the exception may contain information such as password. Upstream
applications that just log the message (as several appear to do)
could inadvertently expose these passwords to a user with read access to
the log files. It is therefore considered prudent to invoke
strutils.mask_password() on the command, stdout and stderr in the
exception. A test case has been added (to oslo-incubator) in order to 
ensure that all three are properly masked.

An earlier commit (853d8f9897f8563851441108a9be26b10908c076) failed
to address ssh_execute(). This change set addresses ssh_execute.

OSSA is aware of this change request.

Change-Id: Ie0caf32469126dd9feb44867adf27acb6e383958
Closes-Bug: #1377981
Merge "Mask passwords in exceptions and error messages" into proposed/juno