Reset requests before calling callbacks

[jamielennox]

When a callback is invoked that calls requests the previous called mock is run. When this is supposed to be a real_http request the NoMockAddress is thrown but it can't be handled in the adapter and so goes back to the outer send function.

This is all a problem and for a really unusual case because almost surely if you are doing a requests call within a requests-mock callback you're trying to do a real requests call. There's no point having a callback that would simply point to something else you registered in the same adapter.

So let's just assume that and reset all mocking whilst the callback code may be run.

Change-Id: I9a426c54af778fcd536925744e78843854320a07

[jamielennox]

Moved from: https://review.openstack.org/#/c/399377/