Add an API extension for creating+deleting flavors

This extension is a step towards deprecating openstackx for horizon.
Most of the extension code is based on the equivalent in openstackx.

v2:
    s/lifecycle/manage/ for all bits
    Address Pádraig style issues
    Drop purge API option
    Adjust now inaccurate comment in DB api
    Make extension admin_only
    Extend existing /flavors namespace rather than os-flavor-lifecycle
    Only allow API access from admin user

v3:
    Some pep8 fixes

v4:
    Adjust to root_gb, ephemeral_gb changes
    Drop admin_only (it's on the way out AIUI)

Change-Id: I3fdfccdd8e7337e1759f5875c3b15fa9954371ef

nova/compute/instance_types.py

@@ -30,9 +30,15 @@ FLAGS = flags.FLAGS
 LOG = logging.getLogger('nova.instance_types')
 
 
-def create(name, memory, vcpus, root_gb, ephemeral_gb, flavorid, swap=0,
-           rxtx_factor=1):
+def create(name, memory, vcpus, root_gb, ephemeral_gb, flavorid, swap=None,
+           rxtx_factor=None):
     """Creates instance types."""
+
+    if swap is None:
+        swap = 0
+    if rxtx_factor is None:
+        rxtx_factor = 1
+
     kwargs = {
         'memory_mb': memory,
         'vcpus': vcpus,