packaging: Show usage when no args is passed to mng domains

This patch adds a check and if no argument is passed, the usage function
is executed causing the print of the help

Bug-Url: https://bugzilla.redhat.com/983245
Change-Id: I8d4ed902971498318b7304d9a4b60164d4056eb1
Signed-off-by: Yair Zaslavsky <yzaslavs@redhat.com>