create frost and grafana read-only user

Closes #23 (closed)

• add ro_username, ro_password and ro_urlto class Thing
• create two seperate read-only users (sta_ro_.../grf_ro_...) because otherwise all FROST views (even the in-between/helper views) are visible in Grafanas respective project datasource when building a dashboard
  • they both share the same password
  • grf_ro_... only has select permissions to tables thing, datastream, observation, journal
• update CreateFrostInstanceHandler.py and CreateThingInGrafanaHandler.py to use the read-only users