Configure 'api_extensions_path' in midonet plugin

'api_extensions_path' need to be set before creating the test api and
instantiate the PluginAwareExtensionsManager in the 'test_db_plugin'
module.

Otherwise, the plugin will raise an exception like:

ExtensionsNotFound: Extensions not found: ['extension_name']

when adding new vendor extensions in the 'supported_extension_aliases'
attribute in the MidonetPluginV2 class.

Change-Id: I666bdadb8cef37d54444e9b55aaab923b6ccf717
Merge "Configure 'api_extensions_path' in midonet plugin"