Add support for parameter_defaults in environment

https://review.openstack.org/#/c/131337/ adds support for a new
section "parameter_defaults" in the environment, so support it
in the client

blueprint env-nested-usability

Change-Id: Ia1f27102f31a42e7b58008a9b4886cde293308c6
Merge "Add support for parameter_defaults in environment"