Add a reasonable to_s method to models

in a workspace with at least one each of: Password, NTLMHash, and SSHKey
 - [ ] msfconsole
 - [ ] `irb`
 - [ ] `Metasploit::Credential::Private.all.map(&:to_s)`
 - [ ] **Verify** it doesn't blow up and that output is reasonable
 - [ ] `Metasploit::Credential::Public.all.map(&:to_s)`
 - [ ] `Metasploit::Credential::Realm.all.map(&:to_s)`