Add --datasource.* CLI flags to replace DATA_SOURCE_* env vars

[nicolastakashi]

Part of #1371.

Summary

GetDataSources() in exporter/datasource.go currently reads the Postgres connection config only from env vars, with no CLI flag path. Add flags that provide the same configuration options, so the env vars have something to deprecate in favor of.

New flags

• --datasource.dsn (replaces DATA_SOURCE_NAME)
• --datasource.uri (replaces DATA_SOURCE_URI)
• --datasource.uri-file (replaces DATA_SOURCE_URI_FILE)
• --datasource.user (replaces DATA_SOURCE_USER)
• --datasource.user-file (replaces DATA_SOURCE_USER_FILE)
• --datasource.pass (replaces DATA_SOURCE_PASS)
• --datasource.pass-file (replaces DATA_SOURCE_PASS_FILE)

[ArthurSens]

Same feedback we gave to #1376: Let's not have sensitive information like database secrets in CLI flags. This can be easily exploited because CLI flags are written to files in the /proc filesystem.