HOT templates get_param allows extra attributes

HOT templates now allow the use of extra attributes for the
get_param function. With this change it is possible to traverse complex
parameters.

If one of the extra arguments does not match with a key or a valid
index, an empty string is returned.

Change-Id: I49f46b164fd21f6cc7dcea8d4ec1046f6cc912b3
Implements: blueprint hot-select