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.
avocado.cli.app: Display --help output on empty args list

Instead of displaying a not very helpful 'too few args'
message, let's display the full --help output on an
empty args list. Comes with a functional test.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado: Display error message and usage on empty 'run' subcommand

Instead of displaying a not very informative 'too few arguments'
message, display the original error message that mentions about
test IDs *and* the run help output. Adjustments to the original
functional test were made accordingly.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
Merge pull request #150 from lmr/more-usability-fixes

More usability fixes