rgw: Allow disabling of sigv4 fix for multisite

Since the server becomes more strict and the client must become more
conformant to go with it, upgrading to this release can cause problems
for multisite configurations.

To provide a workaround, we add the `rgw_sigv4_insecure` option. This
can be set true at the start of an upgrade and set false once it
completes.

Signed-off-by: Adam C. Emerson <aemerson@redhat.com>