Host-level Memory Metrics

This blueprint adds support for host-level memory information to be added to the Nova metrics framework. Specifically, the following metrics:

1) total: the total physical memory available
2) available: the actual amount of available memory that can be given instantly to processes that request more memory in bytes; this is calculated by summing different memory values depending on the platform (e.g. free + buffers + cached on Linux) and it is supposed to be used to monitor actual memory usage in a cross platform fashion

This can then be used by the metrics weigher for scheduling purposes.