Add support for multiple policies

MOM is able to manage multiple facets of a virtualization host at the same time.
For example, it can manage ballooning and ksm.  Today, we just concatenate a ksm
policy with a balloon policy and pass this unified policy into MOM.  A better
solution would be to allow MOM to manage multiple policies at the same time.  A
user could add and remove individual policies as needed.

This can be accomplished by creating a policy dictionary indexed by policy name.
The MOM admin can manage policies individually by name.  At policy evaluation
time, MOM concats the policies in key order and evaluates them the same as it
always has.

Change-Id: I99a501556f535424e6ed0a3e6489918846d40012
Signed-off-by: Adam Litke <agl@us.ibm.com>