Add AUTHORS and ChangeLog into gitignore
Add AUTHORS and ChangeLog into gitignore

AUTHORS and ChangeLog always appear in python-senlinclient.
Add these two files into gitignore.

Change-Id: Idcaaef5f3cf107e4d4e3ef87fe976dc2092c1a1f

openstack/python-senlinclient,openstack/python-senlinclient,stackforge/python-senlinclient,stackforge/python-senlinclient