packaging: spec: requires openssh-clients

Added dependency on openssh-clients, needed for
having ssh command.

Bug-Url: https://bugzilla.redhat.com/1015757
Change-Id: Ic73c38e16a25acef5aefccb52cbf7cd6fa09a101
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>