cephadm: write output files to /etc/ceph by default

cephadm: write output files to /etc/ceph by default

This streamlines the install for most users by putting the config and
keyring files in /etc/ceph, where other ceph CLI commands will find them.

The cost is making isolated installs a bit more painful, by requiring
that users specify a local location for --output-config etc.

Signed-off-by: Sage Weil <sage@redhat.com>