Implement conditional steps execution

[miltolstoy]

Work items:

• initialize conditional steps logic - feat(structure_handler): initialize conditional steps logic #737

• implement artifacts collection - feat(artifact_collector): artifacts collection for conditional steps #716

• handle only one branch step set - feat(structure_handler): allow one branch step to be set in the conditional step #749

• create basic documentation - docs: conditional steps overview #748

• implement report artifacts collection - test(artifact_collector): report artifacts for conditional steps #750

• reformat tests - test: reformat test_conditional_steps.py #790

• add tests for artifacts preprocessing - test(artifact_collector): artifacts preprocessing for conditional steps #791

• implement artifacts & report artifacts preprocessing - feat(artifact_collector): preprocess report artifacts for conditional… #794, fix(artifact_collector): pre-process artifacts and report artifacts in a single step #795

• implement Configuration support instead of Step for branch steps (link) - feat(configuration_support): change conditional branch steps type to Configuration #808, test(configuration_support): test Step->Configuration update for conditional step branches #809

• temporarily disable having children for conditional step - fix(launcher): exit with error if condition step has children #812

• allow other configurations to be launched after the conditional step - fix(structure_handler): execute steps after conditinal step #816

• implement critical flag logic - feat(launcher): explicit warning about 'critical' flag ignoring on co… #817

• temporary disable steps addition - fix(configuration_support): raise explicit error on conditional steps addition #819

• merge into master - feat: conditional steps #820

• fix bugs found - fix(artifacts_collector): fix build initialization if conditional/branch step has no artifacts #825

• implement steps addition and multiplication

• design & implement step filtering for conditional steps (link)

• check work with the if_env_set flag

• check background execution

• check work with TeamCity tags (link1, link2)

• handle config where conditional step has children (link): run child steps before branch steps, use critical flag logic

See discussions and proposals under #636

[miltolstoy]

Preliminary development plan was created during the task discussion:

1. Step(..., if_succeeded=..., if_failed=...)
2. ? Step -> If / IfStep / ConditionalStep
3. ? ConditionalStep(...).then(...).else(...)
4. ? conditional_step >> then | else

"?" steps are subject to discussion.

[i-keliukh]

Children

handle config where conditional step has children
Note that there are several separate design decisions:

• how to deal with children in conditional step itself
• what to do with children in "true branch" and "false branch"
• do we have to put "true branch" and "false branch" into the same structure as conditional step itself?