Add several yaml checks in yamlutils

* Added yaml safe loader
* Ensure the value returned by yaml.load is a dict
otherwise a ValueError is raised. Why this type check?
yaml.load('bla bla') does not raise any exception, it returns
a string.
* Raise a ValueError if the dict returned is empty
* Added Missing Unit Test for yamlutils

Change-Id: I7109248b0a9f88e65e2cf6ee9f77899b1dbc9d8b
Merge "Add several yaml checks in yamlutils"