Don't automatically enable revocation events.

Cuts any direct imports of revoke.model as that
triggers the dependency registration.

In order to fix a dependecy resolution issue, changes the syncronziation to
using the same sort of chaching mechanism as the other drivers.

Adds the ability to Lazy activate providers for future or optional dependency
resolution.

Closes-Bug: 1291099
Related-Bug: 1292283
Change-Id: I0db36b295c2040ec1fb248cf75dc55c44c059211
Replace httplib.HTTPSConnection in ec2_token

httplib.HTTPSConnection is known to not verify SSL certificates in Python 2.x.
Implementation got adapted to make use of the requests module instead.

SSL Verification is from now on enabled by default.

Can be disabled via an additional introduced configuration option:

`keystone_ec2_insecure=True`

SecurityImpact
DocImpact
Partial-Bug: 1188189

Change-Id: Ie6a6620685995add56f38dc34c9a0a733558146a
Merge "Don't automatically enable revocation events."
Merge "Replace httplib.HTTPSConnection in ec2_token"