avocado.plugins.builtin: Builtin plugins are dynamic.

the builtin plugin list were once static coded, but now
they are search and loaded by run-time,
by inspecting the directory <avocado_home>/avocado/plugins.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #149 from ruda/dynamic_loading_builtin_plugins

avocado.plugins.builtin: Builtin plugins are dynamic.