cli: add option to retrieve system summary #854369

[oVirt shell (connected)]# summary

hosts-active          : 0
hosts-total           : 4
storage_domains-active: 3
storage_domains-total : 4
users-active          : 1
users-total           : 3
vms-active            : 0
vms-total             : 7

https://bugzilla.redhat.com/show_bug.cgi?id=854369

Change-Id: I76bc19432b239f8394a15c7e3b37629437d38345
Signed-off-by: Michael Pasternak <mpastern@redhat.com>