<install> fixes and explanation for HA MongoDB

* document the need to run the replica set config step after all
replicants are available
* try to catch error conditions with replica set config
* fix problem with adding port spec to replicants
* remove other "search" entries from /etc/resolv.conf
so that new non-FQDNs have a better chance of resolving
* minor rearrangement of docs, discussion of DNS setup