MSP-9606
In order to support Metasploit::Credential correctly,
metasploit-framework needs to support Metasploit::Concern, which does
all its magic using a Rails::Engine initializer, so the easiest path is
to make metasploit-framework be able to use Rails::Engines. To make
Rails::Engine use Rails::Engine, make a dummy Rails::Application
subclass so that all the initializers will be run when anything requires
msfenv.
