[CHEF-3237] Fix Mac OS X service provider. Do not fail when HOME is not set

* Do not add `~/Library/LaunchAgents` if HOME is not set
* It is safe to assume that if HOME is not set ~ will not expand - see rb_home_dir function in MRI ruby
CHEF-3237 - Incorporated comments from PR 624
CHEF-3237 - Incorporated latest review comments.
CHEF-3237 - Fixed the rspec test.