rgw: making implicit_tenants backwards compatible.

rgw: making implicit_tenants backwards compatible.

In jewel, "rgw keystone implicit tenants" only applied to swift. As of
luminous), this option applies to s3 also.
Sites that used this feature with jewel now have outstanding data that
depends on the old behavior.

The fix here is to expand "rgw keystone implicit tenants" so that it
can be set to any of "none", "all", "s3" or "swift" (also 0=false=none,
1=true=all).  When set to "s3" or "swift", the actual id lookup
is also partitioned.

Formerly "rgw keystone implicit tenants" was a legacy opt.
This change converts it to the new style of option,
including support for dynamically changing it.

Fixes: http://tracker.ceph.com/issues/24348
Signed-off-by: Marcus Watts <mwatts@redhat.com>
(cherry picked from commit a28a38f6e91da3abe59c34fad0e059eeaf29a65f)
(cherry picked from commit d2bdea8e7bc8eae64e4b72f581e424a9a6871aed)

Conflicts:
  - forward port from luminous was easier than backport from master