Try to fix OpenStack dependencies on Py 2.6

setup.py

@@ -85,8 +85,27 @@ def run_tests(self):
         'sqlalchemy<1.2',
     ]
     openstack_requires = [
-        # None, GC3Pie's OpenStack support on Python 2.6 ceased during
-        # the release cycle leading to version 2.5
+        # support for Python 2.6 was removed from `novaclient` in commit
+        # 81f8fa655ccecd409fe6dcda0d3763592c053e57 which is contained in
+        # releases 3.0.0 and above; however, we also need to pin down
+        # the version of `oslo.config` and all the dependencies thereof,
+        # otherwise `pip` will happily download the latest and
+        # incompatible version,since `python-novaclient` specifies only
+        # the *minimal* version of dependencies it is compatible with...
+        'stevedore<1.10.0',
+        'debtcollector<1.0.0',
+        'keystoneauth<2.0.0',
+        # yes, there"s `keystoneauth` and `keystoneauth1` !!
+        'keystoneauth1<2.0.0',
+        'oslo.config<3.0.0',
+        'oslo.i18n<3.1.0',
+        'oslo.serialization<2.1.0',
+        'oslo.utils<3.1.0',
+        'python-keystoneclient<2.0.0',
+        'python-novaclient<3.0.0',
+        'python-cinderclient<1.6.0',
+        # OpenStack's "keystoneclient" requires `importlib`
+        'importlib',
     ]
 elif python_version == (2, 7):
     version_dependent_requires = [
@@ -99,10 +118,7 @@ def run_tests(self):
         'sqlalchemy',
     ]
     openstack_requires = [
-        # The following Python modules are required by GC3Pie's `openstack`
-        # backend. Since OpenStack ceased support for Python 2.6 around
-        # version 3.0.0 of the client libraries, we have to include separate
-        # dependecy lists for Python 2.7+ and Python 2.6
+        'python-keystoneclient',
         'python-glanceclient',
         'python-neutronclient',
         'python-novaclient',