Resolve port conflict when starting multiple PMM agents #10325

Job	Run time
Tests (mysql:5.6, mongo:4.2, postgres:10, percona/pmm-server:2.0.0)	4m 48s
Tests (mysql:5.7, mongo:4.4, postgres:11, percona/pmm-server:2.0.1)	4m 58s
Tests (mysql:8.0, mongo:4.4, postgres:12, percona/pmm-server:2.1.0)	5m 4s
Tests (mysql:8.0, mongo:4.4, postgres:13, percona/pmm-server:2.2.0)	5m 18s
Tests (percona:5.6, percona/percona-server-mongodb:4.2, perconalab/percona-distribution-postgresq...	5m 7s
Tests (percona:5.7, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	5m 29s
Tests (percona:8.0, percona/percona-server-mongodb:4.4, perconalab/percona-distribution-postgresq...	5m 33s
Tests (mariadb:10.2, mongo:4.4, postgres:9.4, perconalab/pmm-server:dev-latest)	4m 57s
Tests (mariadb:10.3, percona/percona-server-mongodb:4.4, postgres:9.5, perconalab/pmm-server:dev-...	5m 17s
Tests (mariadb:10.4, postgres:9.6, perconalab/pmm-server:dev-latest)	5m 0s
	51m 31s

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