Move the salt bootstrap code into cotton.

This still requires some special variables set in the env/project to work. We'll need to tidy up and document the deps for this
Docs for new bootstrap functions
PEP8
Allow install type in bootstrap salt minion.

This lets us do:

    _bootstrap_salt(install_type=“git v0.17.5”)
Merge pull request #16 from ministryofjustice/bootstrap-salt

Move salt bootstrap logic into cotton so that we don't have to do it differently across projects.