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. It is based on this:
https://github.com/rapid7/metasploit-framework/wiki/How-to-Send-an-HTTP-Request-Using-HTTPClient#uri-parsing

## Verification steps:

- [x] Download tomcat: http://tomcat.apache.org/
- [x] Install tomcat: http://tomcat.apache.org/tomcat-7.0-doc/appdev/installation.html
- [x] Make sure the login path is: /manager/html
- [x] Start ```msfconsole```
- [x] ```set rhosts [ip]```
- [x] ```set username [valid user]```
- [x] ```set password [valid pass]```
- [x] ```set STOP_ON_SUCCESS true```, but you don't have to if you don't want to.
- [x] Do ```run```. You should see a successful login, the successful login also indicates the URI was parsed correctly.