Improvement: Fetch and store external plan

[Pierre Padrixe (stannie)]

Fetch a plan hosted remotely, if the end-user POST an assembly with a plan uri having a remote host, we will retrieve the plan and store it in Solum raw_content db field, ideally we should store the plan in Swift (that's why this bug is also tagged 'add-plan-in-swift')

https://github.com/stackforge/solum/blob/master/solum/api/controllers/v1/assembly.py#L82-L85

[Pierre Padrixe (stannie) (pierre-padrixe)]

in heat there is https://github.com/openstack/heat/blob/master/heat/common/urlfetch.py which doesn't download the entire file at once but GET the file chunk by chunk and interrupt the download if the file exceed a defined file size

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/85070