See merge request !119

See https://pypi.org/help/#description-content-type

In some deployments it is important to being able to select only specific
senses on which the promise shall react.

See also slapos@4230cd1b

Tests are rewritten for easier coverage of all cases, including all-ok case.
In the same time code has been improved while detecting inconsistencies.

Sort entry_list by known key, the one most important for the usage.

Instead of hiding unconfigured checks, emit OK message, which gives
information to the user regarding the full status.

The IP list comparision of http_query has been dropped, as the IP presence
shall be compared from DNS result.

Improvements:
 * use real dict in the test
 * merge test cases
 * increase test coverage
 * minimize and improve messages
 * use error state instead of error list
 * always shall full diff during test failure to ease debugging
 * fix ssl_certificate missing information
 * avoid cluttering code with test time

See merge request !113

Also add a "Requires-Python" information so that dropping python2
support will be easier in the future. See https://packaging.python.org/en/latest/guides/dropping-older-python-versions/

Note that this promise is only available in Python3.

We are going to delete instance-gitlab-test.cfg.in from slapos repo,
which is dedicated to testing, now we are going to test the file
which is used in production, aka instance-gitlab.cfg, as a part of
theia resiliency test