sdn: fix OOM issues with ovs-vswitchd on many-core machines
sdn: fix OOM issues with ovs-vswitchd on many-core machines

By default ovs-vswitchd creates #cores+1 pthreads, which each get
the RHEL/Fedora default 8MB of stack.  But for whatever reason,
ovs-vswitchd immediately dirties this region resulting in all 8MB
counted to RSS, leading to excessive memory consumption.

Work around that in two ways:

1) increase the OVS container memory limits
2) limit the number of threads ovs-vswitchd uses on machines
with more than 12 cores

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1571379
Related: https://bugzilla.redhat.com/show_bug.cgi?id=1572797

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