Change SNMP 'hardware.memory.used' metric
to reflect (memTotalReal - memAvailReal) which is the used value
for the memory for a physical machine.
This has to be done as SNMP does not provide a memory metric
for used that can be directly used.
Closes-Bug: #1321537
Change-Id: Ibd8cd38ef15a9e86ccd942412b80e90e3762dd44