PMM-7 Remove redundant ci-default.yml

[ademidoff]

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

Problem
We are managing two similar data structures: one in .gitmodules and one in ci-default.yml. The second one is currently being used by ci.py, which combines it with the config overrides located in ci.yml. It turns out, we can still achieve the goal without ci-default.yml, since the Python script can read as well from the .gitmodules config file.

Solution
Get rid of ci-default.yml, which contains the same data as .gitmodules.

This PR does that and it also refactors the code of ci.py.

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-3774-8757353
Client docker: perconalab/pmm-client-fb:PR-3774-8757353
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-3774-8757353.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-3774-8757353&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-3774-8757353.tar.gz

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/1501/

[BupycHuk]

read_submodules?

[JNKPercona]

Server docker: perconalab/pmm-server-fb:PR-3774-e6af5d5
Client docker: perconalab/pmm-client-fb:PR-3774-e6af5d5
Client tarball: https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-3774-e6af5d5.tar.gz
Staging instance: https://pmm.cd.percona.com/job/pmm3-aws-staging-start/parambuild/?DOCKER_VERSION=perconalab/pmm-server-fb:PR-3774-e6af5d5&CLIENT_VERSION=https://s3.us-east-2.amazonaws.com/pmm-build-cache/PR-BUILDS/pmm-client/pmm-client-PR-3774-e6af5d5.tar.gz

[JNKPercona]

API tests have failed: https://pmm.cd.percona.com/job/pmm3-api-tests/1568/