Prevent access Parameters key in template dict

This is the third patch in order to implement native pseudo parameters
for HOT templates.

If someone needs to access to the Parameters section of a template, now
can use the parameters() method of Template/HOTemplate classes. In order
to enforce this as the only way of doing it, if the Parameters section
is access using the template as a dictionary an exception is raised.

Change-Id: Ia60b4d2393370b12bf2681b0985ce1cd6e91ac1a
Implements: blueprint native-pseudo-params
Merge "Prevent access Parameters key in template dict"