Include test_endpoints.json in source distribution

MANIFEST.in recursively includes .py and .txt files for tests in source
distributions, but not .json files, causing unit tests to fail when run
outside of a git checkout.  This commit fixes that issue.

Downstream bug:  https://bugzilla.redhat.com/show_bug.cgi?id=1072925
Merge pull request #2550 from gholms/test-endpoints

Include test_endpoints.json in source distribution. Fixes #2550.