Correct heat/stack_data template print

Currently flame returns:

  ### Heat Template ###
  ... heat template ...

  None

  ### Stack Data ###
  ... heat template ...

  None

Because TemplateGenerator.heat_template/stack_data_template print
templates and return None instead of returning templates. This change
corrects previous methods to output:

  ### Heat Template ###
  ... heat template ...

  ### Stack Data ###
  ... heat template ...

Change-Id: I9f9ad3e859beba583c9db4a0420ec2dd9c3bc8c9
Merge "Correct heat/stack_data template print"