override rs.members.host with specified names

[yuhanz]

We have the requirement of being able to specify the hostname at each member in a replicaset.

currently, this recipe sets the host name for rs.members.host with the value in node[:fqdn]. but in aws-environment, the host name changes. so we'd like to be able to override it with a static ip. (for some reason, configure through DNS is not an option.)

The proposed solution is the have an override attribute.
members[n]['mongodb']['fqdn_override'] = "10.2.3.4"

such that if this attribute is present, it will be used in place for node['fqdn']