env variables in the avocado.core.settings

[richtja]

Is your feature request related to a problem? Please describe.
Consider environment variables as defaults value for avocado config variables. It's pretty standard in Linux/UNIX applications, and we could add users a possibility to change avocado configuration with environment variables.

This issue is based on discussion in PR #5557 where we were thinking about creating AVOCADO_JOB_RESULTS_DIR environment variable to set datadir.paths.logs_dir for all jobs run under selftests/check.py

Describe the solution you'd like
Define environment variables which will correspond to the avocado config variables.