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 toss a flag on this command that then filters out the private records if asked:

```ruby
desc "Clear an environment's keys from your ssh known_hosts file"
arg_name '<environment>'
command 'ssh-clean-keys' do |c|
  c.switch :private
  c.action do |global_options, options, args|
    raise ArgumentError, "Environment is required" unless args.first
    Opsicle::SSHCleanKeys.new(args.first).execute global_options.merge(options)
  end
end

# lib/opsicle/commands/ssh_clean_keys.rb

module Opsicle
  class SSHCleanKeys
    attr_reader :client
    attr_reader :host_keys

    def initialize(environment)
      @client = Client.new(environment)
    end

    def execute(options={})
      set_host_keys(options[:private])
      # ...
    end

    def set_host_keys(exclude_private=false)
      @host_keys = [:elastic_ip, :public_ip, :public_dns]
      @host_keys.concat([:private_ip, :private_dns]) unless exclude_private
    end
```