Connect 'Package Definitions' with actual murano-api endpoint.

 Support the following operations:
 * get list of packages
 * upload a new package
 * download package
 * delete package

The commit https://review.openstack.org/#/c/84986/ is required.

Change-Id: Ia35f5dd70e9a8b4f352f457e3b2d45b85f177ea7
Partially-implements: blueprint service-defitions-migrate-to-apps-ui
Implement the remaining 'Package Definitions' operations:

* package modification
* package enabling/disabling

Also change ChoiceField in PackageUpload form to MultipleChoicesField.
This commit should be supported with
https://review.openstack.org/#/c/85092/

Change-Id: I95cda3620e4bb355c05247b1a19b30d5e0c12ed4
Partially-implements: blueprint service-defitions-migrate-to-apps-ui