stop libraries being required multiple times

The previous use of require caused File.join on several occasions to
calculate different paths to the same library, depending on which
__FILE__ the library was being calculated as relative to; e.g.
/some/path/prefix/spec/one/bar.rb would do the equivalent of:

  require '/some/path/prefix/spec/one/../../libraries/foo/mylib.rb'

and /some/path/prefix/spec/two/baz.rb would do the equivalent of:

  require '/some/path/prefix/spec/two/../../libraries/foo/mylib.rb'

This would result in mylib.rb being loaded multiple times, causing
warnings from constants being redefined, and worse, multiple objects
representing the same class hierarchy (@@foo) variables.  The latter
actually broke the @@subclasses registration mechanism in
Pacemaker::CIBObject.

By switching to File.expand_path, we ensure we always refer to each
library using a single absolute path, which means Ruby's require
mechanism works as it should, only loading the code the first time
round.

spec/libraries/pacemaker/cib_object_spec.rb

@@ -1,8 +1,8 @@
 require 'spec_helper'
 require 'mixlib/shellout'
 
-require File.join(File.dirname(__FILE__), %w(.. .. .. libraries pacemaker))
-require File.join(File.dirname(__FILE__), %w(.. .. fixtures keystone_primitive))
+require File.expand_path('../../../libraries/pacemaker', File.dirname(__FILE__))
+require File.expand_path('../../fixtures/keystone_primitive', File.dirname(__FILE__))
 
 describe Pacemaker::CIBObject do