enum_ad_* post module fixes

https://github.com/rapid7/metasploit-framework/pull/3175 broke enum_ad_* post modules as it changed the return type from the ADSI queries. 

Fixes: https://github.com/rapid7/metasploit-framework/issues/4387

Poke @OJ @bcook-r7 

## Verification
* Run enum_ad_computers - results should be hashes :(
* Apply PR
* enum_ad_computers, enum_ad_user_comments, enum_ad_service_principal_names and enum_ad_to_wordlist - results should be as expected