Use six compatibility library to load yaml files.
Use six compatibility library to load yaml files.

Previously, python 2.x and python 3 compatibility for loading yaml
files was performed by checking for exceptions.  Standard python
practice is to use the six compatibility library for this purpose.
Code has been updated to use the six library for loading yaml files.
Rearranged imports to comply with OpenStack style guidelines.

Change-Id: I08637ee83daa3fbe28ce1e3795e3b4b9ccd76d54

openstack/tosca-parser,openstack/tosca-parser