Adds two tests to ensure correct behaviour with referencing yaml
anchors and a third test to verify the expansion internally of yaml
anchors and aliases.
* test that anchors are not carried across subsequent top level
invocations of yaml.load(). This will be used subsequently to ensure
that where anchors are allowed across included files they are
correctly reset on each top level call.
* test that where anchors are defined in a top level file and
subsequently included files, duplicate anchors raise exceptions as
though they were defined within the same file.
* test that data returned from yaml loading contains the additional data
specified by the alias. Uses json to force conversion so that the
outputted yaml contains the results of the anchors and aliases instead
of them.
Update documentation to contain details of the use of anchors and
aliases including a refernce to a simple generic example from the
specification as well as a JJB specific example.
Change-Id: I0f2b55e1e2f2bad09f65b1b981baa0159372ee10