Allow swift acls to be deleted.

Allow swift acls to be deleted.

The openstack "swift" command deletes acls by submitting an
acl with an empty string.  The existing logic uses c++ strings,
which can't distinguish between an empty string and a non-existant
string.  Additional, the strings are coming from RGWEnv which
supplies C strings not c++.  Using C strings instead makes it
trivial to pass "non-existance" (as a null string).  It also avoids
some type conversion with string copying.

Fixes: http://tracker.ceph.com/issues/22897
Signed-off-by: Marcus Watts <mwatts@redhat.com>