Use ui-cookiecutter

We can use it as follows:
* To easily generate plugins for beginners.
* In Horizon's startdash command (have to look into this) for internal use.
* To keep plugin compatibility, use generated plugin in integration test.

Action Items:
* Generate ui-cookiecutter project as Horizon's subproject.
* Use it in startdash command
* Update document
* Use it in integration test

ui-cookiecutter[1] was set Ocata-2 milestone at summit[2]

[1]https://github.com/shu-mutou/ui-cookiecutter
[2]https://etherpad.openstack.org/p/horizon-ocata-priorities