Rename --unique-id to --force-job-id

Also improve its documentation, explaining that users should
not use this option unless they know exactly what they're doing

Signed-off-by: Ademar de Souza Reis Jr <areis@redhat.com>
avocado.plugins.runner: Validate --force-job-id

Job ID needs to be a 40 digit hex number. Error out if it's not.

Includes functional tests

Signed-off-by: Ademar de Souza Reis Jr <areis@redhat.com>