Increasing neutron timeout for low memory usage

Increasing neutron timeout for low memory usage

We are noticing several tests failing in our low memory environment
because of timeout in neutron requests.
As an example the test
tempest.api.compute.servers.test_server_actions.ServerActionsTestJSON
fails because it requests to plug a vif, and send request to neutron,
which responds in more than neutron_url_timeout, and since the option
vif_plugging_is_fatal is set to True as default, the test fails.
Shortly thereafter, checking in neutron log you can see the request,
returning with the proper status, after more than neutron_url_timeout,
however, it's already too late once nova already marked the instance
with error status, and so the test fails.

Closes-Bug: #1641135

Change-Id: If0991c114f199490ac0deb71eb569a42d4711359