[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.
Minor changes
Land #2235, @wchen-r7's patch for [SeeRM #6264]
add osvdb ref for openx backdoor
add osvdb ref for chasys overflow
Land #2239, OSVDB refs for Chasys and OpenX
Fix for bug #8297
Fixed getting the policy_hash_list which can fail if elements are null
[SeeRM #89297]
Land #2241 - Fix undefined method `text' for nil:NilClass