This commit continues moving plugins into lib_utils.

This commit does not move any plugins for add-on roles
such as logging and metrics.

Certificates with large serial numbers (such as those from commercial
vendors) previously failed to parse due to absent logic in the
FakeOpenSSLCertificate parser. This would cause the module to abort
and break any other roles depending on the cert expiry checking
results.

* Identifies large serials in HEX format for decimal conversion
* Update unit tests to parse a large serial

Since OpenSSL 1.1.0, default name options ('oneline') for 'x509'
command include 'space_eq'. Explicitly enable 'oneline' in order to
have one behaviour with all versions.

- create pytest fixtures for building certs at runtime
- update tests to use the fixtures
- add tests for load_and_handle_cert
- fix py2/py3 encode/decode issues raised by tests
- add get_extension_count method to fakeOpenSSLCertificate
- avoid using a temp file for passing ssl certificate to openssl
  subprocess
- other test tweaks:
  - exclude conftest.py and tests from coverage report
  - reduce the fail_under to 26%, since the tests being included were
    inflating our coverage