Fix typo in the create_isolated_networks help in config.py

"usable " was mis-spelled as "useable " in tempest/tempest/config.py

Closes-Bug: #1489137
Change-Id: I1af4cdf5c6b1b4080f7f554507d8ed429c309050
diff --git a/tempest/config.py b/tempest/config.py
index 0262d1b..506b7df 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -77,7 +77,7 @@
     cfg.BoolOpt('create_isolated_networks',
                 default=True,
                 help="If allow_tenant_isolation is set to True and Neutron is "
-                     "enabled Tempest will try to create a useable network, "
+                     "enabled Tempest will try to create a usable network, "
                      "subnet, and router when needed for each tenant it  "
                      "creates. However in some neutron configurations, like "
                      "with VLAN provider networks, this doesn't work. So if "