webadmin: Add boot time to Host General tab

Add host's boot time to host's General tab.

Change-Id: I157042522209f5917bcdd951b9259f08cabbd907
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1070348
Signed-off-by: Dima Kuznetsov <dkuznets@redhat.com>