Add option 'openstack_enable_password_retrieve' on charm

[Miguel Meneses]

Enables the instance action “Retrieve password” allowing password retrieval from metadata service

I can see this options appears into 'local_settings.py'
# Setting this to True, will add a new "Retrieve Password" action on instance,
#OPENSTACK_ENABLE_PASSWORD_RETRIEVE = False

But it doesn't exist in the charm itself https://github.com/openstack/charm-openstack-dashboard/blob/master/config.yaml