Add cachemgr.conf.default for easier maintenance

Add cachemgr.conf.default for easier maintenance

cachemgr.conf has for a long while been created conditionally if none
already existed, but no current new default file was added to compare
for new configuration options.

This patch reverts the behaviour to what was reportedy available in 2.5.
Adding a file named cachemgr.conf.default next to the cachemgr.conf

cachemgr.conf.default file is replaces unconditionally so that it remains
current with the most recently installed build.
cachemgr.conf itself is only created if not already present so as not to
loose local configuration changes.