Don't litter the screen with output, so get a handle to /dev/null

Don't litter the screen with the "puppet agent --waitforcert 60 --test"
output, so get a handle to /dev/null
Change-Id: I5f28016086a31091e4819c162e4a911cf800aa81
Bug-Url:https://bugzilla.redhat.com/show_bug.cgi?id=1116775
Signed-off-by: hadong <hadong@redhat.com>