Modify existing gfsh create lucene index command to create the index on an existing region

After GEODE-3930 is done, we will have an internal API that lets us create a lucene index on an existing region (without indexing the data that is present). We should modify the existing LuceneCreateIndexFunction to check to see if the user region already exists. If so, instead of throwing an exception we should go ahead and call the internal API adding in GEODE-3930.

Acceptance:
Calling "create lucene index" on an existing region in gfsh succeeds. After calling create lucene index on an existing region, the index will be useable - new updates will be added to the index, and queries will work. Data that was already in the region before the create index will not be indexed as part of this story.

After calling create lucene index on an existing region, cluster configuration should be updated and new members starting up should create the index.