[FixRM 6264] Check required vulnerable component before testing

tomcat_enum requires the admin web app package for it to work, but by default many Apache Tomcat don't actually have this. The module should check that first before trying usernames.

[FixRM 6264], see:
http://dev.metasploit.com/redmine/issues/6264

I also made changes to do_login in order to verify successful/bad attempts more specific.