Added checks for total disk usage of all mounted disks
Added checks for total disk usage of all mounted disks

Change-Id: I5627303f189f8e78847e39a5fd5bb7f090e12c11
Reviewed-on: http://review.membase.org/5373
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>
Tested-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

vzasade/ns_server,nimishzynga/ns_server,membase/ns_server,pavel-blagodov/ns_server,mschoch/ns_server,pavel-blagodov/ns_server,daverigby/ns_server,t3rm1n4l/ns_server,pavel-blagodov/ns_server,membase/ns_server,membase/ns_server,daverigby/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,membase/ns_server,daverigby/ns_server,ceejatec/ns_server,nimishzynga/ns_server,daverigby/ns_server,nimishzynga/ns_server,t3rm1n4l/ns_server,t3rm1n4l/ns_server,mschoch/ns_server,mschoch/ns_server,nimishzynga/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,membase/ns_server,ceejatec/ns_server,vzasade/ns_server,t3rm1n4l/ns_server,ceejatec/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,ceejatec/ns_server,ceejatec/ns_server,pavel-blagodov/ns_server,vzasade/ns_server,mschoch/ns_server,daverigby/ns_server