examples in CI

[dhodovsk]

trafficstars

Run files in docs/source/examples in our CI, so we can be sure our examples are up to date.

[jpopelka]

i.e. make centos-ci-test target depend on test-doc-examples target

[rpitonak]

Right now if you run make test-doc-examples, if one of the examples fail you cannot see results of others. Proposed solution is to use pytest.