Make the log output less chatty.

This includes the following changes:

- Log messages about adding a DNS record are shown only if the record
  does not exist already and log messages about removing a DNS record
  are shown only if the DNS record exists. This avoids unnecessary log
  chatter when working with static DNS records in a Vagrantfile that
  contains multiple VMs. Previously log messages about those static DNS
  records would have been shown every time a VM was started, now they
  are only shown when starting the first VM.

- Just like most log messages already have been, now log output from the
  DNS resolver logic is prefixed with "[landrush] " as well.

- A new configuration setting "quiet" that if set to true, will hide the
  following messages which are useful on the first day to get insight
  about what's happening under the covers but are pretty useless once
  you understand how landrush works:

  - "Not an OSX machine, so skipping host DNS resolver config."
  - "there are #{DependentVMs.count} VMs left, leaving DNS server and static entries"

  This setting is false by default so that the old behaviour stays the default.