Make ovs optional

Requiring a dev version of ovs here causes version conflict issues
for projects which don't want to use dev versions of libraries. [1]

Make it optional and document it in README instead.

[1] https://bugs.launchpad.net/neutron/+bug/1584858

Signed-off-by: YAMAMOTO Takashi <yamamoto@ovn.org>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>

README.rst

@@ -38,11 +38,13 @@ Some functionalities of ryu requires extra packages:
 - OF-Config requires lxml
 - NETCONF requires paramiko
 - BGP speaker (ssh console) requires paramiko
+- OVSDB support requires ovs  (Note: python 3.4 requires ovs>=2.6.0.dev0)
 
 If you want to use the functionalities, please install requirements::
 
     % pip install lxml
     % pip install paramiko
+    % pip install ovs
 
 
 Support

tools/pip-requires

@@ -5,4 +5,3 @@ oslo.config>=1.15.0
 routes  # wsgi
 six>=1.4.0
 webob>=1.2  # wsgi
-ovs >=2.6.0.dev0