Fix undef comparison in rabbitmq-env.conf template

Conversion from erb to epp introduced a logic error, comparing the value
of the environment variable to the `Undef` Type, rather than comparing
either the type of the value against the `Undef` type, or the value
against the literal `undef`.

Author: optiz0r

templates/rabbitmq-env.conf.epp

@@ -1,5 +1,5 @@
 <% $rabbitmq::config::environment_variables.keys.sort.each |$k| { -%>
-  <%- unless $rabbitmq::config::environment_variables[$k] == Undef {-%>
+  <%- unless $rabbitmq::config::environment_variables[$k] =~ Undef {-%>
 <%= $k %>=<%= $rabbitmq::config::environment_variables[$k] %>
   <%-} -%>
 <% } -%>