Retry timeout to 15sec for etherpad frontend

By default it's 60 secs of timeout whenever the backend application
server goes into an error state. This commit reduces this to 15 secs
allowing for shorter time windows during failure of nodejs backend

Change-Id: If3abc09e492752f1d52b2b8567857d9a1886fa9a
Merge "Retry timeout to 15sec for etherpad frontend" into production