Currently in kingbird-client there is NO support for
PROJECT_DOMAIN_NAME/USER_DOMAIN_NAME OR PROJECT_DOMAIN_ID/USER_DOMAIN_ID
but in the latest release of OpenStack,
there are options for the above-mentioned environment variables.
Added tests for the same and made README.rst better.
Closes-Bug: #1716364
Change-Id: I09625acafc0e3d43fd6008d3de930e98195d14f3