nagios: use hash instead of array for hosts (Bug #3909)

Puppet 2.7.19 introduced a change that don't allow resource titles to be
something else that a string. from the puppet 2.7.19 changelog:

Don’t allow resource titles which aren’t strings

  It was possible to create resources whose titles weren't strings, by
  using a variable containing a hash, or the result of a function which
  doesn't return a string. This can cause problems resolving relationships
  when the stringified version of the title differs between master and
  agent.

  Now we will only accept primitives, and will stringify them. That is:
  string, symbol, number, boolean. Arrays or nested arrays will still be
  flattened and used to create multiple resources. Any other value (for
  instance: a hash) will cause a parse error.

currently, it's much easier to iterate over a hash in puppet than over
an array, cause every resource you call iterating over an array would
need a unique name, and you don't have this in arrays.
"Header set X-Frame-Options: Allow" only for nagios (Bug #4169)

Nagios won't work with setting this option to "DENY",
as set in conf.d/security (#4169). Therefor we allow
it here, only for nagios.
Merge branch 'feature/3909_nagios__Resource_title_must_be' into develop