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
- [ ] Complete Verification Steps for https://github.com/rapid7/metasploit_data_models/pull/27
- [ ]  Install local gem (gem install path_to_gem_file )
- [ ] start up msfconsole
- [ ] use auxiliary/scanner/ssh/ssh_login
- [ ] set RHOSTS 10.6.201.148
- [ ] set username msfadmin
- [ ] set password msfadmin
- [ ] run
- [ ] creds
- [ ] Verify new cred is saved in the db
- [ ] run irb inside msfconsole
- [ ] VERIFY Mdm::Cred.last.task_creds returns an empty array
- [ ] VERIFY Mdm::Cred.last.tasks returns an empty array
- [ ] VERIFY Mdm::Host.last.task_hosts returns an empty array
- [ ] VERIFY Mdm::Host.last.tasks returns an empty array
- [ ] VERIFY Mdm::Services.last.task_services returns an empty array
- [ ] VERIFY Mdm::Services.last.tasks returns an empty array