Allow reindexing to new mapping without service APIs

In https://review.openstack.org/#/c/298976/ we added an optimization to sl-manage index sync such that when one or more resource types are specified they are populated from their service APIs and other resource types in the same resource group are copied from a preexisting index.

There is a further use case where mapping definitions or index settings have changed between deployed versions but source data has not. This might be the case during service upgrades or bug fixes, or ES cluster maintenance. In this case, using the direct index -> index copy for all resource type data is beneficial.

This BP proposes adding a flag --copy-existing-data that will apply the optimization to all resource types. Specifying it with --type will be treated as an error.