Abstract node memory checking into a separate function

The checking of a node's free memory (via rpc.call_node_info) is done in
both start instance an failover. This patch abstracts this call,
together with the appropriate error handling, into a separate function
called _CheckNodeFreeMemory.

The patch also has some related changes:
  - the check is done in prereq and not in exec for start instance
  - the redundant check in exec for failover has been removed

Reviewed-by: ultrotter