Expand template path under user's home directory.

* File.join won't expand `~` into `ENV['HOME']`
  (http://ruby-doc.org/core-1.9.3/File.html#method-c-expand_path)
* The FakeFS File.exists? implementation calls FileSystem#find
  (https://github.com/defunkt/fakefs/blob/master/lib/fakefs/file_system.rb#L22-33)
  containing a call to FileSystem#normalize_path which expands the
  path variable passed in
  (https://github.com/defunkt/fakefs/blob/master/lib/fakefs/file_system.rb#L91-98)
* The file system mocking library sets up a false expectation that `~`
  will be expanded in the #export_template method and consequently the
  production code can't use the template directory
* To guard against future regressions such as fixes/updates to FakeFS or
  using an alternate file system mocking library, the specs were updated
  to explicitly set `ENV['HOME']`

spec/foreman/export/upstart_spec.rb

@@ -38,13 +38,19 @@ describe Foreman::Export::Upstart do
   end
 
   context "with alternate templates from home dir" do
-    let(:default_template_root) {File.expand_path("~/.foreman/templates")}
+    let(:default_template_root) {File.expand_path("#{ENV['HOME']}/.foreman/templates")}
 
     before do
+      ENV['_FOREMAN_SPEC_HOME'] = ENV['HOME']
+      ENV['HOME'] = "/home/appuser"
       FileUtils.mkdir_p default_template_root
       File.open("#{default_template_root}/master.conf.erb", "w") { |f| f.puts "default_alternate_template" }
     end
 
+    after do
+      ENV['HOME'] = ENV.delete('_FOREMAN_SPEC_HOME')
+    end
+
     it "can export with alternate template files" do
       upstart.export("/tmp/init")