Use memoized_method decorator in views

A lot of horizon's views use the pattern:

    def get_data(self):
        if not hasattr(self, "_data"):
            # Calculate data here.
            self._data = data
        return self._data

This is copy-pasted all over the codebase. It's better to handle that
with a single decorator. In the future, we might even replace it with
some better caching approach.

Closes-bug: #1248230
Change-Id: Id021fccf9032e5068993ec91a7774a5d0fbf29bb
Merge "Use memoized_method decorator in views"