[aim-mapping] Auto PTG per L2P

This change automatically creates a PTG per L2P. This PTG is created as a
reverse map of the "default" EPG that is being created per L2P per Neutron
network. We will henceforth refer to this PTG as "auto" PTG.

The ID of the auto PTG is derived from the ID of the L2P as a MD5 hash
calculation (for uniqueness) and persisted in the format:
"auto<hash_of_l2p_id>". It is thus always possible to determine the ID of the
auto PTG from the ID of the L2P and no additional state needs to be maintained.

The initial name of the auto PTG is derived from the ID of the L2P to ease
debugging and troubleshooting, and takes the form: "auto-ptg-<l2p_id>". This
name is mutable (just like any other PTG). The aim_mapping driver does not
have any specical meaning for this name, and does not care about after it
implicitly sets it at the time of the auto PTG creation. Note that changing
the name of the PTG will not result in a change in the display_name of the
default EPG that the auto PTG maps to (since this property is owned by the
apic_aim mechanism driver).

Any Contracts configured directly on the default EPG will be preserved
regardless of any changes to the auto PTG.

The auto PTG cannot be deleted by the end user and doing so will result in
an error.

The shared status of the auto PTG is made consistent with the shared status
of the L2P (once set, it cannot be changed).

The auto PTG is deleted when the corresponding L2P is deleted.

This patch also gets rid of the apic name_mapper's policy_target_group()
function which mapped a policy_target_group's identity to an APIC EPG name.
This is replaced by the function apic_epg_name_for_policy_target_group()
which is now part of the aim_mapping policy driver.

Change-Id: I76977449419fed7984efb534ab88612c88cd3d54
(cherry picked from commit f424a47183)

gbpservice/neutron/plugins/ml2plus/drivers/apic_aim/apic_mapper.py

@@ -156,11 +156,6 @@ class APICNameMapper(object):
     def router(self, session, router_id, router_name=None):
         return router_name
 
-    @mapper(NAME_TYPE_POLICY_TARGET_GROUP)
-    def policy_target_group(self, session, policy_target_group_id,
-                            policy_target_group_name=None):
-        return policy_target_group_name
-
     @mapper(NAME_TYPE_L3_POLICY)
     def l3_policy(self, context, l3_policy_id):
         l3_policy = context._plugin.get_l3_policy(context._plugin_context,