avocado: Make job ID more prominent in avocado

Make some changes to bring the concept of job ID as a
user visible information:

* Make the human output to display the job ID
* Make the json output to have the job ID
* Record the job id in a file inside the results dir

Adapt functional tests to that new schema.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Print better message on Ctrl+C

When the user presses Ctrl+C, print a better message
explaining what is going on.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>