Add requests.Session wrapper class

Wrapper class for requests.Session adds some common OpenStack functionality
* log all requests and responses at debug level
* json-encode request body passed in to request() in json keyword arg
* set default user_agent at Session creation
* set default verify at Session creation

This sets the base for some additional functionality to follow:
* handle redirects properly for a REST API client
* handle a default auth token to set the X-Auth-Token header for simple use cases

Partially implements blueprint: rest-client
Change-Id: Ice3f2dae4a99fe59c7cc384a857900e3ae45a7d2
Merge "Add requests.Session wrapper class"