Do not send blank attributes through API

In Liberty API, sending nil values is not supported anymore.
Fixes for this are being done, but not everywhere. So we should
not send any parameter that was not specified.

lib/fog/openstack/requests/compute/create_server.rb

@@ -6,10 +6,10 @@ def create_server(name, image_ref, flavor_ref, options = {})
           data = {
             'server' => {
               'flavorRef'  => flavor_ref,
-              'imageRef'   => image_ref,
               'name'       => name
             }
           }
+          data['server']['imageRef'] = image_ref if image_ref
 
           vanilla_options = ['metadata', 'accessIPv4', 'accessIPv6',
                              'availability_zone', 'user_data', 'key_name',
@@ -59,15 +59,12 @@ def create_server(name, image_ref, flavor_ref, options = {})
 
           if (block_device_mapping = options['block_device_mapping_v2'])
             data['server']['block_device_mapping_v2'] = [block_device_mapping].flatten.collect do |mapping|
-              {
-                'boot_index'            => mapping[:boot_index],
-                'delete_on_termination' => mapping[:delete_on_termination],
-                'destination_type'      => mapping[:destination_type],
-                'device_name'           => mapping[:device_name],
-                'source_type'           => mapping[:source_type],
-                'uuid'                  => mapping[:uuid],
-                'volume_size'           => mapping[:volume_size],
-              }
+              entered_block_device_mapping = {}
+              [:boot_index, :delete_on_termination, :destination_type, :device_name, :source_type, :uuid,
+               :volume_size].each do |index|
+                entered_block_device_mapping[index.to_s] = mapping[index] if mapping.key?(index)
+              end
+              entered_block_device_mapping
             end
           elsif (block_device_mapping = options['block_device_mapping'])
             data['server']['block_device_mapping'] = [block_device_mapping].flatten.collect do |mapping|