Replace subnetpool config options with admin-only API

This patch adds a new boolean 'is_default' property to subnetpools. This
allows the admin to set the default v4/v6 subnetpools via the API rather
than the existing neutron.conf options - which are deprecated by this patch.

Only one subnetpool per IP family can be set to default.

DocImpact
ApiImpact

Co-Authored-By: Carl Baldwin <carl@ecbaldwin.net>

Change-Id: I5daba2347cfb91fac0b155b2c1b459ee7d9e4505
Closes-Bug: 1501328

etc/policy.json

@@ -22,8 +22,10 @@
 
     "create_subnetpool": "",
     "create_subnetpool:shared": "rule:admin_only",
+    "create_subnetpool:is_default": "rule:admin_only",
     "get_subnetpool": "rule:admin_or_owner or rule:shared_subnetpools",
     "update_subnetpool": "rule:admin_or_owner",
+    "update_subnetpool:is_default": "rule:admin_only",
     "delete_subnetpool": "rule:admin_or_owner",
 
     "create_address_scope": "",

neutron/tests/etc/policy.json

@@ -22,8 +22,10 @@
 
     "create_subnetpool": "",
     "create_subnetpool:shared": "rule:admin_only",
+    "create_subnetpool:is_default": "rule:admin_only",
     "get_subnetpool": "rule:admin_or_owner or rule:shared_subnetpools",
     "update_subnetpool": "rule:admin_or_owner",
+    "update_subnetpool:is_default": "rule:admin_only",
     "delete_subnetpool": "rule:admin_or_owner",
 
     "create_address_scope": "",