Wake up, dynamic plugins, improved discover

• Workaround yaml key sorting on rhel-8 [fix #207]
• Fix test discovery from the execute step scripts
• Merge discover step documentation and fixes [#204]
• Document the discover step, fix issues, add tests
• Simplify the minimal example, adjust tests
• Move fmf_id() to Node class, minor adjustments
• Allow to print fmf identifier in tmt tests show
• Merge manual tests story and examples [#198]
• Add a story and examples describing manual tests
• Sync more extra-* attributes when exporting [#199]
• Enable checks for essential test attributes
• Handle require in Dicovery
• Store imported metadata in a sane order [fix #86]
• Enable Python 3.8 in Travis, update classifiers
• Add missing 'require' attribute to the Test class
• Fix long environment for run.sh [fix #126]
• Merge dynamic plugins and wake up support [#186]
• Implement dynamic plugins and options [fix #135]
• Suggest using 'tmt init' when metadata not found
• Merge improved import of tier from tags [#187]
• Adjust tier import from test case tags
• Merge tmt test export --nitrate --create [#185]
• Adjust suppport for creating new nitrate testcases
• Allow creation of nitrate cases when exporting
• Create tier attribute from multiple Tier tags
• Fix run.sh to work with RHEL/CentOS 7 as well
• Implement wake up for Run, Step and Discover