PMM-12619: Make PMM environment variables more consistent #11714

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, perconalab/pmm-server:3-dev-latest)	3m 30s
Tests (mysql:5.7, mongo:4.4, postgres:11, perconalab/pmm-server:3-dev-latest)	3m 34s
Tests (mysql:8.0, mongo:4.4, postgres:12, perconalab/pmm-server:3-dev-latest)	3m 30s
Tests (mysql:8.0, mongo:4.4, postgres:13, perconalab/pmm-server:3-dev-latest)	3m 40s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	3m 31s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	3m 31s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	3m 44s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:3-dev-latest)	3m 36s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:3-de...	3m 40s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:3-dev-latest)	3m 30s
	35m 46s

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