Adjust audit logs to avoid negative mem/cpu info

we might have following audit info in the log:
AUDIT nova.compute.resource_tracker [-] Free ram (MB): -1559
AUDIT nova.compute.resource_tracker [-] Free disk (GB): 29
AUDIT nova.compute.resource_tracker [-] Free VCPUS: -3

which is really confusing to operater and useless
This patch adjust the log to print phy info and virtual cpus.

DocImpact

Change-Id: I2bc63e1aae0787bf976660eed8fd8899bfffe593
Closes-Bug: #1316916
Merge "Adjust audit logs to avoid negative mem/cpu info"