Add missing spec for Metasploit::ERD::Diagram::DEFAULT_OPTIONS[:inheritance]

MSP-9816
Add default filetype of png

MSP-9816
Metasploit::ERD::Clusterable

MSP-9816

Rename Metasploit::ERD::Domain to Metasploit::ERD::Clusterable and add
 #diagram alongside #domain.  #diagram allows uses to simply do
entity.diagram.create to create a png ERD in the current directory.