Drop os.common.exceptions

The os.common.exceptions lib was deprecated in oslo and all usages should be
removed.

We're only using os.common.exceptions.Error:

class Error(Exception):
    def __init__(self, message=None):
            super(Error, self).__init__(message)

and we have our own base exception class with all needed stuff
savanna.utils.exceptions.SavannaException.

Change-Id: Ib4a1601e4b71bb38d293d457afaffa1c979b8ffb
Closes-Bug: #1208734

openstack-common.conf

@@ -5,7 +5,6 @@ module=setup
 module=jsonutils
 module=xmlutils
 module=timeutils
-module=exception
 module=gettextutils
 module=log
 module=local

savanna/exceptions.py

@@ -13,10 +13,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-import savanna.openstack.common.exception as ex
 
-
-class SavannaException(ex.Error):
+class SavannaException(Exception):
     """Base Exception for the project
 
     To correctly use this class, inherit from it and define