Feature/task associations/cred service host

PT Story: https://www.pivotaltracker.com/story/show/49167601
Related PRs: https://github.com/rapid7/metasploit_data_models/pull/27
https://github.com/rapid7/pro/pull/720

This adds the backend support to creating the Task association objects for Host,Service, and Cred when reporting on those objects. In a pure framework context this won't do anything since framework doesn't use tasks.

Verification steps
- [x] Complete Verification Steps for https://github.com/rapid7/metasploit_data_models/pull/27
- [x]  Install local gem (gem install path_to_gem_file )
- [x] start up msfconsole
- [x] use auxiliary/scanner/ssh/ssh_login
- [x] set RHOSTS vuln-metasploitable2
- [x] set username msfadmin
- [x] set password msfadmin
- [x] run
- [x] creds
- [x] Verify new cred is saved in the db
- [x] run irb inside msfconsole
- [x] VERIFY Mdm::Cred.last.task_creds returns an empty array
- [x] VERIFY Mdm::Cred.last.tasks returns an empty array
- [x] VERIFY Mdm::Host.last.task_hosts returns an empty array
- [x] VERIFY Mdm::Host.last.tasks returns an empty array
- [x] VERIFY Mdm::Services.last.task_services returns an empty array
- [x] VERIFY Mdm::Services.last.tasks returns an empty array