Ability to use external ClickHouse database instance

User story:

As a user, I want to use external Clickhouse instance.

Acceptance criteria:

• We can use a separate Clickhouse instance outside the container
• We can setup clickhouse hostname/port and database with environment variables.

Out of scope:

• Changing settings from UI (hostname, login/password, and so on)
• TLS + authorization

Suggested implementation:

 -

How to test:

We can use external Clickhouse from outside of the container. Clickhouse setting can be changed with environment variables: 

PERCONA_TEST_PMM_CLICKHOUSE_ADDR -> hostname:port

PERCONA_TEST_PMM_CLICKHOUSE_DATABASE -> database name
PERCONA_TEST_PMM_CLICKHOUSE_POOL_SIZE -> https://github.com/ClickHouse/ClickHouse/blob/master/programs/server/config.xml#L1130
PERCONA_TEST_PMM_CLICKHOUSE_BLOCK_SIZE -> https://clickhouse.com/docs/en/operations/settings/settings/#setting-max_block_size
 

1. QAN works as expected with external Clickhouse
2. QAN works as expected with internal Clickhouse
3. clickhouse datasource works as expected