Unique indices and validations that include realm_id

[MSP-10963](https://jira.tor.rapid7.com/secure/RapidBoard.jspa?rapidView=84&view=detail&selectedIssue=MSP-10963)

# Pre-verification steps
- [ ] Release https://github.com/rapid7/metasploit-credential/pull/69
- [ ] Update `Gemfile` and `Gemfile.lock` to released metasploit-credential

# Verification Steps

## msfconsole

- [ ] `./msfconsole --environment development`
- [ ] `irb`
- [ ] `Metasploit::Credential::Core.destroy_all`
- [ ] `exit`
- [ ] `creds add-password user password realm`
- [ ] VERIFY `creds` shows added password: 
```msfconsole
Credentials
===========

host  service  public  private   realm  private_type
----  -------  ------  -------   -----  ------------
               user    password  realm  Password
```
- [ ] `creds add-password user password`
- [ ] VERIFY `creds` shows credential with and without realm: 
```msfconsole
Credentials
===========

host  service  public  private   realm  private_type
----  -------  ------  -------   -----  ------------
               user    password         Password
               user    password  realm  Password
```

## specs

- [ ] `rake spec`
- [ ] VERIFY no failures