Make rpm plugin call sh -c'...'

The rpm plugin is the last user of shell syntax. For now have it
call its pipeline explicitly using sh -c '...'.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>
Call Popen with shell=False

Fixes Issue #253.

Signed-off-by: Bryn M. Reeves <bmr@redhat.com>