zuul: prefix server default template with 'zuul.'

Will introduce a default for zuul-merger. So should prefix them
properly.

Change-Id: Ief4d919a47deaa3a608f79c06d705bed770e4a5d
zuul: merger now has its own default file

And make puppet reload zuul-merger whenever the default change.  The
configuration still points to zuul.conf but I will craft a custom one
later on.

Change-Id: Ife1a15f72ffd3fff322f5ff75740a91ff551b5d3