docs.source.WritingTests: Introduce environment variables.

Add new subsection about environment variables that
Avocado exports to the tests.

Plus fix some typos.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #129 from ruda/environmenmt_variables_documentation

docs.source.WritingTests: Introduce environment variables.
Add multiplexer.py.
add multiplexer_unittest.py
Remove multiplex_config_unittest.py
Remove multiplex_config.py
Beginning the merge of the new multiplexer.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
tests.sleeptest: Using YAML file to multiplex.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
tests.synctest: Update multiplex file to the YAML.

Signed-off-by: Ruda Moura <rmoura@redhat.com>