avocado.plugins.manager: Fix wrong variable to log.

Fix the variable to display in log when an external plugin fails
to load.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
selftests: Add  functional tests regarding external plugins.

Add functional tests to explore external plugins in two
bad scenarios: 1) plugin without configure() method implemented,
2) plugin with syntax error.

Signed-off-by: Ruda Moura <rmoura@redhat.com>
Merge pull request #157 from ruda/fix_small_bug_external_plugins

Fix small bug in external plugins