Remove py26 support from designateclient

as of mitaka, the infra team won't have the resources available to
reasonably test py26, also the oslo team is dropping py26 support
from their libraries. sine we rely on oslo for a lot of our work,
and depend on infra for our CI, we should drop py26 support too.

Change-Id: Id322ec683643c817bef1de43519394808cc64072
Closes-Bug: #1519510

setup.cfg

@@ -16,7 +16,6 @@ classifier =
     Programming Language :: Python
     Programming Language :: Python :: 2
     Programming Language :: Python :: 2.7
-    Programming Language :: Python :: 2.6
     Topic :: Internet :: Name Service (DNS)
 
 [global]