Fix #3975 - Register TARGETURI, not URI

This fixes https://github.com/rapid7/metasploit-framework/issues/3975

The module should register TARGETURI and call #target_uri for URI validation.

## Verification steps:

- [ ] Download tomcat: http://tomcat.apache.org/
- [ ] Install tomcat: http://tomcat.apache.org/tomcat-7.0-doc/appdev/installation.html
- [ ] Make sure the login path is: /manager/html
- [ ] Start ```msfconsole```
- [ ] ```set rhsots [ip]```
- [ ] ```set username [valid user]```
- [ ] ```set password [valid pass]```
- [ ] ```set STOP_ON_SUCCESS true```, but you don't have to if you don't want to.
- [ ] Do ```run```. You should see a successful login, the successful login also indicates the URI was parsed correctly.