The new heat 'hiera' hook is incompatible with the old
tripleo-puppet-elements hiera element. The two formats
should not coexist as they will overwrite each others
config files (last hook to run would win).
To make the end user experience nice we add an explicit check
here. Note: this would require us to clean out old
apply-config data from a prior run during an upgrade to make
sure the check doesn't mistakenly prevent a new deployment
On python <3.4 the separator for json.dump() defaulted to (', ', ': ')
which may cause trailing whitespaces in rendered JSON files when used
with indent. This change sets the separator to (',', ': ') to prevent
trailing whitespaces with older versions of python.