A common issue when running rb-site install in an automated setup (for

Docker instances or when using a tool like Chef) is that each new

install would get its own SECRET_KEY setting, requiring users to

override the resulting file.
To address this, a new --secret-key option has been added, which

allows the caller to provide an explicit key to use. This must be 50

characters or higher (matching what we generate ourselves). If this is

set, it will be used instead of generating a new key.

Tested creating a new site with and without --secret-key.
Tested the character length validation.