Skip test_subnet_details slow test until bug 1813198 is fixed

The bug is killing throughput in our CI system [1] so until it's
fixed I'm proposing that we skip it.

http://status.openstack.org/elastic-recheck/#1813198
http://status.openstack.org/elastic-recheck/gate.html#1813198

Change-Id: I1a6f35dcb15110297cd491b902d7a8279861997e
Related-Bug: #1813198
1 file changed