Give auth_brute ability to try credentials stored in db

Added two options:
DB_USER_PASS: this will try each user/pass couple stored in the db
DB_ADD_ALL: this will add each user and password to the lists.
By setting this to true, auth_brute will try every user with
every known password.
Removed options DB_ADD_ALL. Added options DB_ALL_PASS and DB_ALL_USERS
to add already known user and passwords to the lists.
Add tentative fixes
Merge pull request #1 from wvu-r7/pr/1215

Tentative fixes for rapid7/metasploit-framework#1215
Land #1215, creds reuse for AuthBrute modules