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. Conversely, log messages about removing a DNS
  record are shown only if the DNS record exists.  This especially
  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, log output from the DNS
  resolver logic are prefixed with [landrush] as well now.

- A new configuration setting "verbose" that if set to false, 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 true by default so that the old behaviour stays the default.