(maint) Quote class name in init.pp template

According to the style guide:

All resource titles should be quoted. (Puppet supports unquoted resource
titles if they do not contain spaces or hyphens, but you should avoid
them in the interest of consistent look-and-feel.)

A class is a resource.
Merge pull request #2673 from roidelapluie/init_quote_class

(maint) Quote class name in init.pp template