MB-4786: use sigar memory stats for UI server memory usage gauge

We're now sending mem_total and mem_free with heartbeats and node info
now prefers them to erlang's memory info metrics.

Change-Id: Iaa7cb7d868da54043871f4de5ad78beaae62000d
Reviewed-on: http://review.couchbase.org/27859
Tested-by: Aliaksey Kandratsenka <alkondratenko@gmail.com>
Reviewed-by: Aliaksey Artamonau <aliaksiej.artamonau@gmail.com>