Fix a syntax error in stow.yaml that broke the 'python_use_stow' option. While
at it, make sure that a proper error message is shown if no stow environments
are installed on the system.
Change-Id: Ifa696daa367997fb705ba22527137f4f051ca558
map returns a generator that needs to be converted to a list for
the loop.
We also don't need unique as this is iterating over a list of
directory names that must already be unique.
Change-Id: Ibd22d79be29aaa9d3a7924319c59929e665f9cbc
In case image is prepared with python-stow-versions element,
it make sense to use stow to just enable required python version instead
of building it for each job run.
Change-Id: Ie04a2bb59d351c1e5c79b2da79f7a094c44cdf86
