Allow ehcache.xml to be overidden

This is a backport of issue #1012 for the 2.1.x branch.

ehcache.xml needs a rename to ehcache-default.xml

https://github.com/playframework/playframework/pull/993/files#L15R132

Alternatively ehcache.xml could simply be renamed to ehcache-failsafe.xml as EhCache already provides such fallback behaviour.