Pin down OpenStack package versions for Python 2.6

requirements.openstack.txt

@@ -1,5 +1,25 @@
 # The following  Python modules  are required by  GC3Pie's `openstack` backend.
 #
-python-novaclient
-# required on Python 2.6: recent versions of OpenStack modules use it
-importlib
+python-novaclient;python_version>='2.7'
+# Alternate dependencies for Python 2.6:
+# - pyCLU requires argparse,
+argparse;python_version=='2.6'
+# - OpenStack's "keystoneclient" requires `importlib`
+importlib;python_version=='2.6'
+# - 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;python_version=='2.6'
+debtcollector<1.0.0;python_version=='2.6'
+keystoneauth<2.0.0;python_version=='2.6'
+# yes, there's `keystoneauth` and `keystoneauth1` !!
+keystoneauth1<2.0.0;python_version=='2.6'
+oslo.config<3.0.0;python_version=='2.6'
+oslo.i18n<3.1.0;python_version=='2.6'
+oslo.serialization<2.1.0;python_version=='2.6'
+oslo.utils<3.1.0;python_version=='2.6'
+python-novaclient<3.0.0;python_version=='2.6'