Lint all files
Add upgrade notice about nagios::nsca::server

Move upgrade notes above Overview section to make them more obvious
require main package to be installed before all config files

Not having those files generates lots of errors. Among those are some
such as:

err: /Stage[main]/Nagios::Base/File[nagios_confd]/ensure: change from
absent to directory failed: Could not set 'directory on ensure: Could
not find group nagios at /etc/puppet/modules/nagios/manifests/base.pp:83

but other problems arise when trying to place config files when the
configuration directory doesn't even exist. As a side effect to this,
after the first puppet run, nagios doesn't start up since some config
files might be missing.

All of the config files have links like "notify => Service['nagios']"
and one would think that this would define the correct dependency order,
but apparently it does not.
main icinga config file is named icinga.cfg, not nagios.cfg
Point the main config file to an icinga-specific file

There are some configurations in that file that are icinga-specific, so
we need to diverge from the nagios configuration. If we use the same
configuration file as the one used by nagios, the icinga service will
just not start.

There actually was already a default icinga configuration file so let's
just point the resource to the right sources so that this
icinga-specific default file will be used.