[cern] Set coredns cache to 900 secs

Spyridon Trigazis requested to merge os10031 into cern/train

The DNS service comlained to users for high load. Per docs, let's put it to 15 mins. https://service-dns.web.cern.ch/faq.asp#caching https://coredns.io/plugins/cache/

Change-Id: I88fe0fc168f516f933ebab59eaa30019de00ab06 Signed-off-by: Spyros Trigazis spyridon.trigazis@cern.ch

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