PMM-7 API tests rework

[maxkondr]

https://jira.percona.com/browse/PMM-7

percona/pmm#5299

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4329-66d488c
Client docker: perconalab/pmm-client-fb:PR-4329-66d488c
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4329-66d488c
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4329-66d488c.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4329-66d488c&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4329-66d488c.tar.gz

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/5953/

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-4329-bedc250
Client docker: perconalab/pmm-client-fb:PR-4329-bedc250
Watchtower docker: perconalab/pmm-watchtower-fb:PR-4329-bedc250
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4329-bedc250.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-4329-bedc250&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-4329-bedc250.tar.gz

[JNKPercona]

API tests have succeded: https://pmm.cd.percona.com/job/pmm3-api-tests/5970/