Automatic SECURE_KEY generation

The SECURE_KEY of every Django installation should be set to a unique value upon deployment. Currently, it is the system administrators responsibility to change this to a secure (i.e. unique) value. He has several ways to achieve this:

- Do it manually (current state)
- Use a (Linux) distribution-specific package that generates a key in a post-installation step
- Deploy via crowbar with a modified dashboard barclamp that generates the key

The first option is tedious and error-prone (and easily forgotten). The second option fails if the package is part of a pre-generated appliance (thus installed only once but deployed multiple times) and the latter is beyond our control. Instead, we should allow dashboard instances to generate the secret key automatically once upon first start. However, this can be rather tricky, as the typical Apache+mod_wsgi+daemonized deployment involves several Python interpreters accessing the code, so there's some kind of locking needed. Here's a possible implementation:

  https://github.com/saschpe/horizon/compare/bp/automatic-secure-key-generation

It has the advantage that SECRET_KEY doesn't have to be part of local_settings.py anymore and thus is one thing less one needs to care for. It has an additional dependency on the 'lockfile' Python module, but I don't see this as an issue, as it is also pulled in by nova (as a transitive dependency of python-daemon).