Add dependencies between SSL certificate steps

If (for example) the key file is 0 bytes long and is unable to generate a key
the previous arrangement would fail on the CSR creation. Now it will skip the
following steps rather then failing everything and it can be manually
investigated laster.

In the example above delete the empty key and
--start-at 'Install DKIM senders domain lookup file' will generate it next time
around.
Adding Fail2ban integration

This integration provides everything short of actually setting up f2b which
can't be done in this role. The required filter is provided and the
installation smarts along with a new variable to set if desired.