Build static dashboards

[DifferentialOrange]

This patch add several make targets to build a static dashboard.

make DATASOURCE=MyPrometheus JOB=MyApp \
     OUTPUT_STATIC_DASHBOARD=mydashboard.json build-static-prometheus

Following targets are available:

• build-static-prometheus: Tarantool dashboard for a Prometheus datasource;
• build-static-tdg-prometheus: TDG dashboard for a Prometheus datasource;
• build-static-influxdb: Tarantool dashboard for an InfluxDB datasource;
• build-static-tdg-influxdb: TDG dashboard for an InfluxDB datasource.

Variables for Prometheus targets:

• DATASOURCE: name of a Prometheus data source;
• JOB: name of a Prometheus job collecting your application metrics;
• RATE_TIME_RANGE (optional, default 2m): rps computation rate time range;
• OUTPUT_STATIC_DASHBOARD (optional, default dashboard.json): compiled dashboard file.

Variables for InfluxDB targets:

• DATASOURCE: name of a InfluxDB data source;
• POLICY (optional, default autogen): InfluxDB metrics retention policy;
• MEASUREMENT: name of a InfluxDB measurement with your application metrics;
• OUTPUT_STATIC_DASHBOARD (optional, default dashboard.json): compiled dashboard file.

This patch also adds tests to run these targets.

Closes #90

[DifferentialOrange]

I strongly suggest to review this PR commit by commit.