Fixed server bootstrap to remove default image if a boot disk is specified

lib/fog/google/compute.rb

@@ -138,7 +138,7 @@ module Fog
         end
 
         def backoff_if_unfound(&block)
-          retries_remaining = 5
+          retries_remaining = 10
           begin
             result = block.call
           rescue Exception => msg

lib/fog/google/models/compute/servers.rb

@@ -51,6 +51,11 @@ module Fog
             :public_key_path => File.expand_path("~/.ssh/id_rsa.pub"),
             :username => ENV['USER'],
           }
+          if new_attributes[:disks]
+            new_attributes[:disks].each do |disk|
+              defaults.delete :image_name if disk['boot']
+            end
+          end
 
           server = create(defaults.merge(new_attributes))
           server.wait_for { sshable? }