Optimize provisioning mechanism of plugins, dashboards, and datasources in PMM

Let's do short investigation what and how we can  (if we need to .. at all ) change our dashboards from API  to http://docs.grafana.org/administration/provisioning/ 

Looks like this will be more stable way to load dashboards 

Please note - we shop dashboards inside PMM application so need to maintain this  + have fixed folders for our dashboards

What's happens:

We want to move dashboard provisioning from  import-dashboard.py to the internal Grafana mechanism and remove import-dashboard.py:

https://grafana.com/docs/grafana/latest/administration/provisioning/

How to test:

1. Check that all dashboard work as expected.
2. Check that upgrade from the old version works as expected and dashboard works after upgrade (Docker and UI upgrade)
3. Check those dashboards fit in the right folders.