Fix vault token file access.

Fix vault token file access.

Put the vault token file in a location that ceph can read.
Make it readable only by ceph.

On rhel8 (and indeed, any vanilla rhel machine), $HOME is liable to be
mode 700.  This means the ceph user can't read things in that user's
directory.  This causes radosgw to emit the confusing message "ERROR:
Vault token file ... not found" even though the teuthology log will
plainly show it was created and made readable by ceph.

Fixes: http://tracker.ceph.com/issues/51539
Signed-off-by: Marcus Watts <mwatts@redhat.com>