Adds cred_counter to Hosts

## PT #48595211
- [x] In mdm dir, `gem build metasploit_data_models.gemspec`
- [x] cd pro/ui && gem install ./metasploit_data_models.gem && rake db:migrate
- [x] Run pro specs
- [x] Add separate framework DB: https://github.com/rapid7/pro/wiki/MDM-gem
- [x] Alter this for your path: `gem install PATH_TO_MDM_DIR/metasploit_data_models.gem`
- [x] Run framework specs
- [x] Pro functional test: start prosvc, rails, check a workspace with hosts that have creds or import the file attached to https://www.pivotaltracker.com/story/show/48699101: `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.
- [x] MSF functional test: start msfconsole, connect to the dev DB, connect to workspace with creds, run `hosts` and `creds`, no errors, should see cred entries.