Adds cred_counter to Hosts

## PT #48595211
- [ ] Edit pro Gemfile to point to local path for MDM gem: https://github.com/rapid7/pro/wiki/MDM-gem
- [ ] cd pro/ui && rake db:migrate
- [ ] Run pro specs
- [ ] Add separate framework DB: https://github.com/rapid7/pro/wiki/MDM-gem
- [ ] Edit framework Gemfile to point to local path for MDM gem
- [ ] Run framework specs
- [ ] Functional test: start prosvc, rails, check a workspace with hosts that have creds: `select cred_count from hosts where id = X`  Or create a new workspace, a new host, manually add a cred, run the command above to verify counter.