Document the options for HA setups

After many questions on the users list about this it was agreed that it would be nice to document how ODS can be used in a high availability deployment. Dave Knight sent a presentation on how ICANN do this: https://www.dns-oarc.net/files/workshop-201005/ha-opendnssec-oarc.pdf

There are two main aspects to capture. One is that a feed of the configuration files and db are required to keep two signers in sync since key selection is not completely deterministic. The other is that it is possible to trigger the enforcer to sign at specific times (as opposed to intervals that originate from when the daemon is started) by signalling it.