Incubation:APM Credentials as secrets in deployment

Currently credentials to access ClickHouse are various connection strings. These should use secrets that are mounted into container filesystems and then loaded into the process separately.

Specifically:

• Gateway clickhouse username/password (scoped to read/write to apm database)
• Migrations clickhouse username/password (scoped to admin of apm database)
• ClickHouse operator username/password (specific maintenance permissions)