Ship with etc/oslo.rootwrap instead of etc/oslo

Ship default configuration file in etc/oslo.rootwrap to encourage best
packaging practices (packagers should deploy rootwrap.conf in
/etc/oslo.rootwrap rather than /etc/oslo/rootwrap or /etc/oslo).

Also change default filters_path directory from
/etc/oslo.rootwrap/rootwrap.d to /etc/oslo.rootwrap/filters.d
as it avoids redundancy and is more self-explanatory.

Change-Id: Iadf623571283cba1886bb27a5f55c59ba66c522f