Break out NoDevice property validation (Fixes #1551)

* NoDevice is a string for launch template block device mapping and
  a null object for EC2 block device mapping
* This addresses the issue by breaking out block device mapping to
  two separate objects. The EC2 version will keep the original name
  for backwards compatibility purposes and the launch template
  version will take up a new name

troposphere/ec2.py

@@ -138,8 +138,17 @@ class BlockDeviceMapping(AWSProperty):
     props = {
         'DeviceName': (basestring, True),
         'Ebs': (EBSBlockDevice, False),  # Conditional
-        'NoDevice': (dict, False),
         'VirtualName': (basestring, False),  # Conditional
+        'NoDevice': (dict, False)
+    }
+
+
+class LaunchTemplateBlockDeviceMapping(AWSProperty):
+    props = {
+        'DeviceName': (basestring, True),
+        'Ebs': (EBSBlockDevice, False),  # Conditional
+        'VirtualName': (basestring, False),  # Conditional
+        'NoDevice': (basestring, False)
     }
 
 
@@ -975,7 +984,7 @@ class LaunchTemplateCreditSpecification(AWSProperty):
 
 class LaunchTemplateData(AWSProperty):
     props = {
-        'BlockDeviceMappings': ([BlockDeviceMapping], False),
+        'BlockDeviceMappings': ([LaunchTemplateBlockDeviceMapping], False),
         'CpuOptions': (CpuOptions, False),
         'CreditSpecification': (LaunchTemplateCreditSpecification, False),
         'DisableApiTermination': (boolean, False),