Add namespace_id to project creation for a user in API

This patch just adds the possibility to change the namespace of a
project created for a user via the API.

It behaves like https://github.com/gitlabhq/gitlabhq/pull/3119.

It might be interesting, for an administrator, to create projects in
the global namespace for specific users. This is possible when we create a
project without specifying the owner so why not here.