[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 form, 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: mytest
    desc: "Mytest description"
    maintainer: lmr

    input:
        args:
            param1:
                - 100
                - 500
                - 1000
            param2:
                - 10
                - 50
                - 100
            param3:
                - foo
                - bar
                - baz


Current files for existing tests were changed.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>