Add some EPG-related functions to PolicyManager

Added functions to
- get the vnid of an end-point group
- check if a group exists

Also made other getXXXForGroup functions thread-safe.

Change-Id: I550d9c279ff19bba0cdbccaa3d6145b048f58340
Signed-off-by: Amit Bose <bose@noironetworks.com>