Fix utils.execute() for consistency with Oslo code

Oslo processutils.execute() can use non-string command line
arguments, they convert to strings internally.
This patch fix utils.execute(), non-string arguments can be
used as in Oslo execute without error.

Change-Id: I4f640582375b244e23a6641c3b94157c49c8ee3b
Merge "Fix utils.execute() for consistency with Oslo code"