PMM-12619: Make PMM environment variables more consistent #11712

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, perconalab/pmm-server:3-dev-latest)	6m 33s
Tests (mysql:5.7, mongo:4.4, postgres:11, perconalab/pmm-server:3-dev-latest)	6m 16s
Tests (mysql:8.0, mongo:4.4, postgres:12, perconalab/pmm-server:3-dev-latest)	6m 20s
Tests (mysql:8.0, mongo:4.4, postgres:13, perconalab/pmm-server:3-dev-latest)	6m 26s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	6m 32s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	6m 32s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	6m 32s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:3-dev-latest)	6m 30s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:3-de...	6m 19s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:3-dev-latest)	6m 34s
	1h 4m 34s

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