Change how main.cf and master.cf are generated

Previously main.cf and master.cf Postfix configuration files were
generated by templating separate config parts in
/etc/postfix/{main,master}.cf.d/ directories on remote hosts and
assembling them into /etc/postfix/{main,master}.cf files afterwards.

This approach has certain advantages and disadvantages: configuration
process takes longer (lots of files to generate and move between Ansible
Controller an remote hosts), but external roles or local administrators
can put their config files separately in
/etc/postfix/{main,master}.cf.d/ directories and they will be
automatically picked up by Ansible during assembly. You can also very
easily mess up the service configuration by overwriting variables
defined previously and only manual intervention on remote host can fix
that. There's also no easy way to remove obsolete configuration parts
from remote hosts.

Because of that I'm changing the way Postfix role manages these files.
Instead of assembling them on remote hosts, Ansible will template
/etc/postfix/{main,master}.cf on Ansible controller from parts via Jinja
include directives and send them in one piece to remote hosts.

This makes Postfix configuration tasks much faster and less error-prone.
Other roles or local admins lose the ability to directly put
configuration in the {main,master}.cf config files, but instead they
should use debops.postfix dependent role variables as a public API
interface. That way Postfix role can manage its config files correctly
and merge all relevant variables for multiple roles at once.
Move task condition logic to tasks/main.yml
Update tasks to use YAML syntax instead of inline
Use full FQDN names instead of variable names
Add an 'archive' capability in Postfix

If 'archive' capability is enabled on a Postfix server, it configures
transport map and {sender,recipient}_bcc_maps to automatically add blind
carbon copy address to each message passing through the SMTP server.
This in effect creates a copy of the message which is sent to an archive
server (which needs to be configured separately).

Address of the archive account and server to send messages to is by
default configured automatically. If 'local' mail is enabled, archived
messages will be directed to 'mail-archive' account on the SMTP host. If
'local' mail is disabled, archived mail messages will be sent to
'mail-archive' account to a server on local domain with 'archive.'
subdomain. Both of these (archive server with a DNS subdomain and mail
account) need to be configured separately by the sysadmin.

You can also change how 'archive' capability behaves using several
variables configured via inventory. You can change the destination mail
address, specify how archived mail is sorted (if at all) and only
archive mail from or to specific domains, among other things.
Merge pull request #2 from drybjed/postfix-rewrite

Clean up Postfix tasks, add archive capability, modify main.cf/master.cf