Remove empty string defaults from class parameters

In preparation for puppet 4, the puppet-lint-empty_string-check gem
helps find where empty strings are assigned to variables because how
empty strings are evaluated changes in puppet 4, so the easiest thing
is to just remove usage of them.

More importantly, it is not appropriate to set defaults for parameters
that are, realistically, required. If the users leaves these parameters
blank, puppet will apply most resources successfully but fail
mysteriously on the Exec['run_accessbot'] resource, causing the user to
spend additional time debugging. If no defaults are provided in the
puppet class, puppet fill fail with a message stating that the user
must supply values. This "fail fast" approach is much preferred to
debugging obtuse python tracebacks.

This change is technically backwards-incompatible. However, it will not
break Infra because we are already supplying these parameters via
system-config/modules/openstack_project/manifests/eavesdrop.pp. If
there are other users using this module and they are not passing in
these parameters, then their accessbots cannot possibly be running.

An alternative approach is to supply undef as parameter defaults. This
will allow the puppet-lint gem to pass, but does not help the user at
all.

Change-Id: I046340b852e7e8983b741e5bad415678977bea0d

manifests/init.pp

@@ -1,10 +1,10 @@
 # == Class: accessbot
 #
 class accessbot(
-  $nick = '',
-  $password = '',
-  $server = '',
-  $channel_file = '',
+  $nick,
+  $password,
+  $server,
+  $channel_file,
 ) {
 
   user { 'accessbot':