Add helper to clear ssh host keys for an environment

Add helper to clear ssh host keys for an environment.

See: #57 

It clears the _ip or _dns keys it finds on the instance.

Hiding the private IPs behind an argument came to mind, but I thought doing so would conflict with the goal of it being a quick / dirty way of cleaning things out.

Alternate PR that skips the private IPs: sportngin/opsicle#59

```
$ opsicle --verbose ssh-clean-keys staging
Executing: ssh-keygen -R x.x.x.1
/Users/user/.ssh/known_hosts updated.
Original contents retained as /Users/user/.ssh/known_hosts.old
Executing: ssh-keygen -R x.x.x.2
/Users/user/.ssh/known_hosts updated.
Original contents retained as /Users/user/.ssh/known_hosts.old
Executing: ssh-keygen -R ec2-x-x-x-x.compute-1.amazonaws.com
/Users/user/.ssh/known_hosts updated.
Original contents retained as /Users/user/.ssh/known_hosts.old
Executing: ssh-keygen -R 10.x.x.x
/Users/user/.ssh/known_hosts updated.
Original contents retained as /Users/user/.ssh/known_hosts.old
Executing: ssh-keygen -R ip-10-x-x-x.ec2.internal
/Users/user/.ssh/known_hosts updated.
Original contents retained as /Users/user/.ssh/known_hosts.old
```
I would do all this fun jazz in a helper method:

```ruby
HOST_KEYS = [:elastic_ip, :public_ip, :public_dns, :private_ip, :private_dns]

def execute(options={})
  instances.each do |instance|
    instance_records(instance).each do |host|
      # Is this properly escaped against expansion?
      command = "ssh-keygen -R #{host}"
      Output.say_verbose "Executing: #{command}"
      system(command)
    end
  end
end

def instance_records(instance)
  HOST_KEYS.map { |key| instance[key] }
           .compact.uniq
end
```