I would like to test juju charms on trystack.org, however I have not found any docu, how to configure juju for the openstack API?
First off you need juju and charm-tools, ensure you have backports enabled (they are enabled by default) to get the latest Juju:
sudo apt-get install juju-core charm-tools
Here's an example OpenStack configuration for
Other OpenStack Based Clouds:
This answer is for generic upstream OpenStack support, if you're using an OpenStack-based provider check these questions out for provider-specific information: