Reduce Stack complexity to below 20

This patch reduces the complexity of Stack module to be below 20 by
further splitting the handle_update method.
Since this is a pure code refactor work, no test case modification is
needed.

Change-Id: I92e58d749dedd32c73b9ea1d5b2e54be84f69003
Convert unicode to six.text_type in heat/tests

This patch converts unicode to six.text_type in heat/tests
for python 3.x compatibility.

Change-Id: I2e7553ee3486e03c6a9bc0c2a022f99d6298257b
Convert unicode to six.text_type in heat/api

This patch convert unicode to six.text_type in
heat/api for python 3.x compatibility.

Change-Id: I8a1f88ebaac64daa2ba074407fa682dff00f2371
Convert unicode to six in heat/engine/resources

This patch converts unicode to six.text_type in
heat/engine/resources for python 3.x compatibility.

Change-Id: I967e70b9e9fa017479461b7f1c2baf7c2870c70c
Merge "Convert unicode to six.text_type in heat/tests"
Merge "Convert unicode to six.text_type in heat/api"
Merge "Convert unicode to six in heat/engine/resources"
Merge "Reduce Stack complexity to below 20"