Integrate MON and Grafana

Add actions for datasources in mon charm

create, list and delete prometheus datasources

requests are done using grafana API calls

grafana-url, grafana-admin, grafana-password does
not have default value. If they are not present in
config the charm is blocked.

Change-Id: Ia0138b8d3088654f65ea9d23a664619a4475d3d8
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>