Move mock to test-requirements

So that the spec will not need mock any more.

.travis.yml

@@ -5,6 +5,7 @@ python:
 # command to install dependencies
 install: 
   - "pip install -r requirements.txt"
+  - "pip install -r test-requirements.txt"
   - "pip install pep8 --use-mirrors"
   - "pip install nose --use-mirrors"
 # command to run tests

requirements.txt

@@ -1,6 +1,5 @@
 pbr
 jsonrpclib
 netaddr
-mock
 cliff
 oslo.config>=1.6.0

test-requirements.txt

@@ -0,0 +1 @@
+mock