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 ensure that all three are
properly masked.

An earlier commit (e8199714) failed to address ssh_execute(). This
change set addresses ssh_execute.

OSSA is aware of this change request.

Change-Id: I0db9e98cbeb2a5e6f9ae0074f24717aa91cfc238
Closes-Bug: #1343604