Load linux_admin once when needed

Due to some concerns about loading `linux_admin` multiple times when
calling `MiqSystem.num_cpus` causing a memory leak, this creates a
helper method that no-ops after it has been required once.

lib/gems/pending/util/miq-system.rb

@@ -47,7 +47,7 @@ def self.cpu_usage
 
   def self.num_cpus
     return unless Sys::Platform::IMPL == :linux
-    require 'linux_admin'
+    require_linux_admin
     @num_cpus ||= LinuxAdmin::Hardware.new.total_cores
   end
 
@@ -228,6 +228,12 @@ def self.open_browser(url)
     when :mingw, :mswin then `start "#{url.gsub('"', '""')}"`
     end
   end
+
+  def self.require_linux_admin
+    return true if @linux_admin_required
+    require "linux_admin"
+    @linux_admin_required = true
+  end
 end
 
 if __FILE__ == $0