Add API support for flavors

This patch adds the control API for flavors. The patch allows users to
create, delete, update and list flavors. This API is considered an
admin-only API, hence it's registered as part of the private endpoints.

DocImpact

Partially-Implements blueprint marconi-queue-flavors

Change-Id: Id29d29940f2ecabab0531edefe018c0dd2f39811
Merge "Add API support for flavors"