External components (Clickhouse,PG, VM) improvements

External Postgres issues:

1. PERCONA_TEST_POSTGRES_USERNAME but PERCONA_TEST_POSTGRES_DBPASSWORD. I think it should be PERCONA_TEST_POSTGRES_PASSWORD

2. Why PMM tries to use pmm-managed database as default? Database with such name can’t be created

External Clickhouse improvements:

1. Allow possibility to use external Clckhouse with username and password, now we have the only next variables for Clickhouse PERCONA_TEST_PMM_CLICKHOUSE_BLOCK_SIZE, PERCONA_TEST_PMM_CLICKHOUSE_DATASOURCE_ADDR,  PERCONA_TEST_PMM_CLICKHOUSE_ADDR, (btw why do we have 2 these variables?) PERCONA_TEST_PMM_CLICKHOUSE_DATABASE, PERCONA_TEST_PMM_CLICKHOUSE_POOL_SIZE

And it was found that if PMM uses all external components, all internal components are still running (retest with env vars needed here to confirm).