This patch adds "--env" argument, which allows one to specify
[path:]key:value records which are injected into the multiplex tree
parsed from yaml files.

It also demonstrate the way plugins can override multiplex tree
therefor modify test params during activate/run phase.

There are few intrusive changes. It reuses the existing namespace
during "avocado.core.parser.Parser.finish" part to keep the values set
during "Plugin.activate" phase and it adds
"self.args.default_multiplex_tree" argument which is reserved for
multiplex tree to be merged into tree made from yaml files.
Signed-off-by: NLukáš Doktor <ldoktor@redhat.com>