Move oslotest to test only requirements

oslotest is only used for unit testing, so
it shouldn't be in requirements.txt

Change-Id: I5073fbd93712072cec8598663590e35fc2edd08d

requirements.txt

@@ -30,6 +30,5 @@ websockify>=0.5.1,<0.6
 wsgiref>=0.1.2
 oslo.config>=1.2.0
 oslo.rootwrap
-oslotest
 pycadf>=0.5.1
 oslo.messaging>=1.3.0

test-requirements.txt

@@ -11,5 +11,6 @@ pylint==0.25.2
 python-subunit>=0.0.18
 sphinx>=1.2.1,<1.3
 oslosphinx
+oslotest
 testrepository>=0.0.18
 testtools>=0.9.34