[RFC] avocado.multiplex_config: Drastically simplify multiplex config

ATTENTION: DO NOT MERGE INTO MASTER.

Since I've struggled to get the hierarchical form of the multiplex config, I decided to take a break, and while doing so, it occurred to me we could use a simpler approach to the problem. So here it goes:

Simplify the multiplex config, by using YAML, such as
other test/task automation such as Travis CI and taskotron.
The new config file would look like:

    ---
    name: synctest
    desc: Runs the synctest test suite
    maintainer: Lucas Meneghel Rodrigues <lmr@redhat.com>

    input:
        args:
            loop:
                - 10
                - 50
                - 100
            sync:
                - 100
                - 500
                - 1000

Current files for existing tests were changed. A real example of how that would work:

    $ scripts/avocado run synctest --multiplex tests/synctest/synctest.yml 
    DEBUG LOG: /home/lmr/avocado/logs/run-2014-05-28-19.51.12/debug.log
    TOTAL TESTS: 9
    (1/9) synctest.loop_10.sync_100:  PASS (2.61 s)
    (2/9) synctest.loop_10.sync_500:  PASS (2.16 s)
    (3/9) synctest.loop_10.sync_1000:  PASS (2.42 s)
    (4/9) synctest.loop_50.sync_100:  PASS (2.35 s)
    (5/9) synctest.loop_50.sync_500:  PASS (2.16 s)
    (6/9) synctest.loop_50.sync_1000:  PASS (2.26 s)
    (7/9) synctest.loop_100.sync_100:  PASS (2.87 s)
    (8/9) synctest.loop_100.sync_500:  PASS (2.61 s)
    (9/9) synctest.loop_100.sync_1000:  PASS (2.63 s)
    TOTAL PASSED: 9
    TOTAL ERROR: 0
    TOTAL FAILED: 0
    TOTAL SKIPPED: 0
    TOTAL WARNED: 0
    ELAPSED TIME: 22.07 s


Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>