Add 'apt_preferences' role

This patch removes APT preferences support from 'apt' role and puts it
in a separate role, which can be called as a dependency by other roles
('nginx' role is the first one to utilize it this way).
Make lists of hosts in firewall unique

This change will filter out duplicates from host lists in 'ferm' and
'tcpwrappers' roles.
Update CHANGELOG.md
Add 'apt_preferences' support to 'redis' role
Merge pull request #128 from drybjed/role/apt

Add 'apt_preferences' role, small bugfixes