* Move transport customizations to their own module
This helps to accomplish a few things:
1) Makes the forced user-agent customization on HTTP requests a bit
clearer by subclassing the targeted objects (as opposed to hiding the
behavior behind a forced override of the google_auth_httplib2 object
methods)
2) Standardizes the creation of HTTP objects. These objects can still
largely be customized, but using a single creation mechanism will
standardize a default and streamline creation, thereby decreasing the
code that would otherwise be replicated in the caller
3) Moves create_http() to a more general purpose module, since it will
likely be used by more than gapi-related methods.
* Use string values for TLS version tests
More closely matches [existing
behavior](4fb73e6073/src/var.py (L853))
that sets the global default to a string value.
