Add custom pre and post Leapp snippets

[jakduch]

Summary

• render optional custom pre/post snippets around the Leapp commands
• indent upgrade snippets as Ansible tasks and keep missing snippets a no-op
• document the snippet names and cover both job template providers

Testing

• 3 template render tests, 4 assertions
• rubocop --parallel (45 files, no offenses)
• ERB syntax checks

Fixes #164