upstream CI: Simplify pipelines enviroment creation

Several optimizations have been done to the pipelines, to make them
closer to what can be reproduced, with the existing scripts, in a
development environment:

- Use start.sh and build.sh scripts to build and start containers
- Use variables to configure different stages instead of using separate
  files
- Use a commom 'prepare_environment' to create the environment for every
  pipeline
- Use a single file defining testing steps (run_tests.yml)
- Remove Centos 7 pipelines
- Reduce the number of pipelines in the test matrix due to the amount of
  time that tests were using
- Use Azure "loop" (each) to create test groups

The above changes make the pipelines easier to understand and modify.

infra/azure/build-containers.yml

@@ -15,16 +15,6 @@ pool:
 
 stages:
 
-# Currently, it's not possible to use CentOS container
-#
-# - stage: CentOS_7
-#   dependsOn: []
-#   jobs:
-#   - template: templates/build_container.yml
-#     parameters:
-#       job_name_suffix: Centos7
-#       distro: centos-7
-
 - stage: CentOS_8_Stream
   dependsOn: []
   jobs: