avocado.settings: Make convert_value_type() more generic

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.test: Log received test parameters

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.utils.params: Convert value type when specified in multiplex cfg

When we specify the type of a variable in the multiplex config:

    sleep_length = 1.0
    sleep_length_type = float

Now we'll convert automatically the string 1.0 retrieved
from the multiplex system to a float in a transparent way.
This will save code by making people to avoid to manually
convert values (if not specified, the value is considered
a string).

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
tests/sleeptest: Convert sleep_length type automatically to float

Tell the multiplex config file we're using floats as
the sleep_length value.

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