Add python-pytun to test-requirements.

python-pytun was installed via tox_install.sh. It is better to install
it via test-requirements.txt and the tox I/S rather than manually.

Change-Id: I9c57334da120ec4291888e5a8a4369fe11f82853

test-requirements.txt

@@ -6,6 +6,7 @@
 hacking<0.11,>=0.10.0
 
 coverage>=3.6 # Apache-2.0
+python-pytun>=2.2.1 # MIT
 python-subunit>=0.0.18 # Apache-2.0/BSD
 sphinx!=1.2.0,!=1.3b1,<1.3,>=1.1.2 # BSD
 oslosphinx!=3.4.0,>=2.5.0 # Apache-2.0

tools/tox_install.sh

@@ -41,6 +41,5 @@ fi
 
 pip install -U $*
 pip install python-etcd
-pip install python-pytun
 pip install scapy
 exit $?