Fixes unicode problems in p35

In py35 the unicode function was removed so I adopted the
handling seen in
https://github.com/openstack/glance/blob/master/glance/common/exception.py#L57-L61

Change-Id: I2d234b5447ad030ac3996834d89857f28a3537c7

octavia/amphorae/driver_exceptions/exceptions.py

@@ -12,6 +12,8 @@
 # License for the specific language governing permissions and limitations
 # under the License.
 
+import six
+
 from oslo_utils import excutils
 
 
@@ -31,8 +33,7 @@ class AmphoraDriverError(Exception):
                     super(AmphoraDriverError, self).__init__(self.message)
 
     def __unicode__(self):
-
-        return unicode(self.msg)
+        return six.text_type(self.msg)
 
     @staticmethod
     def use_fatal_exceptions():

octavia/common/exceptions.py

@@ -17,6 +17,8 @@
 Octavia base exception handling.
 """
 
+import six
+
 from oslo_utils import excutils
 from webob import exc
 
@@ -46,7 +48,7 @@ class OctaviaException(Exception):
                     super(OctaviaException, self).__init__(self.message)
 
     def __unicode__(self):
-        return unicode(self.msg)
+        return six.text_type(self.msg)
 
     @staticmethod
     def use_fatal_exceptions():