dashboard: set datasource and datasource parameters dynamically

[DifferentialOrange]

Set metrics datasource and datasource parameters (like job, measurement and policy) through dynamic Grafana variables. job/measurement is considered to be a potential source of tarantool metrics if it has tnt_info_uptime metric.

After this PR, it is possible to import dashboard artifacts through provisioning/dashboard API.

If this pull request introduces support for new metrics, do not forget to

• add Grafana panel;
• [x] run make update-tests to build new dashboard artifacts;
• add example application code to generate non-null metrics;
• [x] attach a screenshot of a new panel to the PR description;
• add new Telegraf tag key, if relevant,
  • to all telegraf.conf files,
  • to "Grafana dashboard" documentation page;
• add alert example, if relevant,
  • to alerts.yml,
  • to "Alerting" documentation page;
• update supported_metrics.md.

In any case, do not forget to

• [x] link an issue,
• [x] add a CHANGELOG entry.

Closes #185