Bump octavia-lib requirement to 3.8.0

To support the VIP security groups patch, we need to bump the minimum
required version of octavia-lib to 3.8.0

Change-Id: I7a3d7fb6ba1d5b32071b590a79fb039a465fe5fd

Author: johnsom

requirements.txt

@@ -45,7 +45,7 @@ castellan>=0.16.0 # Apache-2.0
 tenacity>=5.0.4  # Apache-2.0
 distro>=1.2.0 # Apache-2.0
 jsonschema>=3.2.0 # MIT
-octavia-lib>=3.6.0 # Apache-2.0
+octavia-lib>=3.8.0 # Apache-2.0
 setproctitle>=1.1.10 # BSD
 python-dateutil>=2.7.0 # BSD