twister: allow loading external configuration

Add a new option to Twister that allows to load alternative
configuration files, `--alt-config-root`. This new option takes an
arbitrary number of root folders where alternative configuration files
can be stored. Twister will check if a test configuration file exists in
any of the alternative test configuration folders. For example, given
`$test_root/tests/foo/testcase.yaml`, Twister will use
`$alt_config_root/tests/foo/testcase.yaml` if it exists.

This feature can be useful if an out-of-tree project needs to run
upstream tests in different configurations, potentially not available
upstream (e.g. an out-of-tree board, or Kconfig setting).

Note that overlaying has been discarded because we can't easily delete
fields from the original configuration file, something that in certain
cases could be required. Current approach may lead to some dupplication,
but guarantees full control of the test configuration.

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>