Rename attrs_exist() to contains_allowed_attrs()

attrs_exist() checks the attributes in the given list against the
attributes in the expected_attrs list. The name expected_attrs is
misleading since based on the name one would expect the given list
to contain all the attributes in the expected list but thats not
what this function does. Based on this, the following changes were
made:

- renamed attr_exist() to contains_allowed_attrs() which checks
  whether all the attributes in the given list is present in the
  allowed_attrs list.

Change-Id: Ie169271e211aa93c1bd3f64a2723aa1cd03a56e9
Closes-Bug: #1285964