Add no-trace on config, test_set and scenario level

[man1207]

1. Add possibility to define tracer: off in run.yml config

2. Add possibility to define tracer: off in config.yml at test_set level

3. Add possibility to define tracer: off in scenario.yml at scenario.yml level

4. Add description into tracing.md docs

Some examples:
Example 1. Disable tracer on certain scenarios. Set tracer: off on those scenarios where tracing is not needed.
Example 2. Your sipssert consists of multiple test sets. You need to disable tracing in some test set as a whole for all scenarios inside that test set. Set tracer: off in config.yml on test set level. Tracer will be disabled for all scenarios inside that test set.
Example 3. You need to disable tracing on global level. Set tracer: off in run.yml file. Remember, that tracing as a whole will be disabled. No one scenario will be traced even if you force tracer: on in scenario.yml due to global tracing disabling priority

[razvancrainea]

Amazing, thanks a lot for your contributions!