webadmin: Add Reboot VM action button

Added new button for Reboot VM to webadmin and userportal.

This command should be enabled only when the GuestAgent is
present because that is currently the only supported means
of reboot in VDSM.

Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1051451
Change-Id: I4a30dad6f2d3b909457e39ac15510f0468c2b1a1
Signed-off-by: Martin Betak <mbetak@redhat.com>