avocado.multiplex_config: Drastically simplify multiplex config

Simplify the multiplex config, by using YAML, such as
other test/task automation such as Travis CI and taskotron.
The new config file looks 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>
.travis.yml: Update file with PyYAML dependencies

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