.fmf/config allow to redefine tree root relatively

[LecrisUT]

For example:

root: ../tests

This would be helpful for projects that want to consolidate the configurations outside the git root.

Issues:

• detecting the .fmf folder when navigating the tree files.
  Although this could be handled by specifying --root, it could be helpful to expose the navigation api via entry-points. E.g. a default implementation could be to read the closest pyproject.toml file and extract the root from there, but in non-python projects, alternative logics could be installed from an external fmf plugin e.g. fmf[github]
• recursiveness
  This one could be problematic when fmf is able to read hidden folders, but since .fmf name is fixed, it should be easy to handle
• ambiguity if multiple .fmf are found
  E.g. if the current directory has a .fmf folder and there is a main .fmf in the repository is desired. This is an ambiguity for the current structure as well, but if this can be customized from the plugins, this could resolve the ambiguity by the choice of installed/enabled plugins, e.g. if a fmf[toml] is enabled, look for a top-level pyproject.toml before navigating the file tree. This could also help with making tmt options be read from user/system config files