allow l3 flavors to provide extensions

[YAMAMOTO Takashi]

some of l3 flavors provide extensions.
eg. "dvr" flavor provides "dvr" extension.
eg. "midonet" flavor would provide "fip64" extension.

for in-tree stuff (dvr, ha, etc) extensions are currently hardcoded in
L3RouterPlugin.supported_extension_aliases.
user requests are validated runtime. (_ensure_driver_supports_request)

on the other hand, for out-of-tree stuff (eg. midonet/fip64) currently there's no way for l3 flavors
to express support.

while we can add all of possible extensions to L3RouterPlugin.supported_extension_aliases,
probably it's more appropriate to introduce a way for l3 flavors to provide extensions, which can be used by both of in-tree and out-of-tree flavors.

[Armando Migliaccio (armando-migliaccio)]

At the PTG we discussed this briefly, and the suggestion was to work around the issue for midonet by inheriting/overriding the list of supported_extension_aliases in the midonet L3 plugin. We should probably refrain from providing the extension mechanism for L3 flavors at this point.