NiFi-Registry 0.5.0 and Bugfixes

• Updated NiFi-Registry Client to 0.5.0
• It is now possible to have group level policies applied instead of just user
• Corrected security.bootstrap_policies for Registry Proxy commands
• Updated docker helper versions to latest
• Implemented utils.rest_exceptions to wrap normal Rest exception raising
• Modified Linting ignore list to remove noise
• Linting fixes for relevant complaints
• Corrected Exception testing to reflect tidied up exception raising
• Version pegging changes to continue support for Py2 and Py3 until deprecation of Py2 in 2020