Cache improvements, copy nodes, modify metadata...

• Implement the new 'fmf clean' command
• Functions to manipulate cache
• Fetch the whole tree using utils.fetch_tree()
• Raise FileException on duplicate YAML keys
• Allow to set desired cache location
• Adjust support for storing modified data to disk
• Add support for storing modified metadata to disk
• Prevent exploring tests in the tmp directory
• Simplify special distro comparison examples
• Store command output in run(), fetch() exceptions
• Use filelock for locking, expand test coverage
• Adjust locking for Tree.node() and utils.fetch()
• Use locking to checkout/read tree
• Enable integration testing with tmt, some cleanup
• Implement a new method copy() for cloning nodes
• Adjust fetch() with custom destination and env
• Support calls fetch(destination, env) and run(env)
• Use the origin default branch instead of master
• Store commit hash if the tree is under a git repo