Add support for before and after request functions
Add support for before and after request functions

In flask it is possible to register functions that will run before
and after every request, these are useful for ensuring certain
things occur (setting up environments) and protecting against
cases such as un-enforced API calls not explicitly white-listed.

Partial-Bug: #1776504
Change-Id: I0138a4676c1e257cd0c238bbe11a520446c299cb

openstack/keystone,openstack/keystone,mahak/keystone,mahak/keystone,mahak/keystone,openstack/keystone