Add plugin logger to rubocop allowed global vars

The plugin logger that is generated should be added to the list of
allowed global variables in the `.rubocop_local.yml` config file.

Author: agrare

lib/generators/manageiq/plugin/templates/.rubocop_local.yml

@@ -0,0 +1,3 @@
+Style/GlobalVars:
+  AllowedVariables:
+  - $<%= plugin_short_name %>_log