Use filesystem paths overlays in `file_roots` to provide target pillar data (instead of symlinks in source repositories)

[uvsmtid]

There is no need to create symlinks within source profile pillars repository ("overrides") and all other repositories ("commons" and "defaults"). This can be achieved using filesystem paths overlays by configuring file_roots in Salt master (or masterless minion) via configure_salt.py script.

The benefits are:

• No noisy diff (or meld) output containing also target profiles during comparison.
• Cleaner repositories (no symlinks automatically generated by configure_salt.py).

Make sure to update populate_initial_project_repositories.sh which will require no bootstrap subdirectories in all pillars.

[uvsmtid]

Cross-reference to umbrella task #8.

[uvsmtid]

It's done.

Commit a30bd34404869e5510d5311f3fdf440f56d91975 merges working Salt master auto-configuration (using configure_salt.py script) using filesystem overlays to provide bootstrap target profile. More specifically, the changes to the relevant scripts are done in commit 760f2a27b0a5266a3fe7d1aba487a184dbacdc73.