replace httplib.HTTPSConnection in EC2KeystoneAuth

httplib.HTTPSConnection is known to not verify SSL certificates
in Python 2.x. This change replaces use of httplib.HTTPSConnection
with the requests module. It imports config settings related to SSL
verification: ssl.key_file, ssl.cert_file, and ssl.ca_file. It also
adds one config setting: keystone_ec2_insecure. By default, SSL
verification is on, but can be disabled by setting:

keystone_ec2_insecure=true

This patch is based on the keystone middleware ec2 token patch:

https://review.openstack.org/#/c/76476

SecurityImpact
DocImpact
Closes-Bug: #1373992

Change-Id: I8e46d41164e9478b820cad569ba82f25de244620
Merge "replace httplib.HTTPSConnection in EC2KeystoneAuth"