make net/ssh require optional

lib/fog/digitalocean/models/compute/server.rb

@@ -81,7 +81,6 @@ module Fog
 
         def setup(credentials = {})
           requires :ssh_ip_address
-          require 'net/ssh'
 
           commands = [
             %{mkdir .ssh},

lib/fog/glesys/models/compute/server.rb

@@ -90,7 +90,6 @@ module Fog
 
         def setup(credentials = {})
           requires :ssh_ip_address, :username
-          require 'net/ssh'
 
           attrs = attributes.dup
           attrs.delete(:rootpassword)

lib/fog/joyent/compute.rb

@@ -1,6 +1,5 @@
 require 'fog/joyent/core'
 require 'fog/joyent/errors'
-require 'net/ssh'
 
 module Fog
   module Compute
@@ -126,6 +125,12 @@ module Fog
           end
 
           if options[:joyent_keyname]
+            begin
+              require "net/ssh"
+            rescue LoadError
+              Fog::Logger.warning("'net/ssh' missing, please install and try again.")
+              exit(1)
+            end
             @joyent_keyname = options[:joyent_keyname]
             @joyent_keyphrase = options[:joyent_keyphrase]
             @key_manager = Net::SSH::Authentication::KeyManager.new(nil, {