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 "Replace httplib.HTTPSConnection in ec2_token"