Honor REQUESTS_CA_BUNDLE env var in Endpoint class

This allows users to set the path to a cert bundle.
The logic is:

* Any explicit value passed to get_endpoint() is used
* Otherwise use REQUESTS_CA_BUNDLE if the var exists in the env
* Otherwise, default to true.
Add verify arg to get_endpoint in Service object
Merge branch 'honor-cacert-envar' into develop

* honor-cacert-envar:
  Add verify arg to get_endpoint in Service object
  Honor REQUESTS_CA_BUNDLE env var in Endpoint class