Update and publish developer documentation for Murano

Developer docs for Murano are slightly out of date [0]. There is no Jenkins job which would build dev-docs and make sure they're 'build-able'. There is no Jenkins job which would publish dev-docs to [1]. We can use Sahara developer docs as an example [2].

Tasks:
* setup infrastructure for automatic doc build and publishing
* fix doc build in Murano (there should be a command 'tox -e docs'
* Add developer documentation. Particular topics will be described in work items

[0] http://git.openstack.org/cgit/stackforge/murano-api/tree/doc/source
[1] http://murano-api.readthedocs.org/en/latest/
[2] https://sahara.readthedocs.org/en/latest/#developer-guide