New app: vm_templates

I decided to split GWM into one more app: Virtual Machine templates.
Previously much of its code was in virtualmachines app and in ganeti_web
app.

Now it's all moved to separate Django application.

Now off to make tests work!
Resolved circular imports in models.

I helped myself with this specific notion of FK in Django:

    blah = models.ForeignKey("app.ModelName")

everywhere.  At last there's some consistency now.
The only exception are Django models like User, Group or ContentType.
Those are imported and then passed through to the FK field.