Fix --network-wide flag in WP-CLI index command

[felipeelia]

Description of the Change

As stated in #2766, ElasticPress 4.x introduced a change in the index WP-CLI command that it would always index all sites, even when the --network-wide flag is not passed. This PR changes that back to the 3.x behavior:

In order to index all sites, the user needs to run wp elasticpres index ... --network-wide, otherwise only the main site will be indexed.

In addition to that, this PR also fixes all WP-CLI commands that rely on get_index_names() method, using indices names for all sites only when it is network activated. Commands affected are:

• get-mapping
• get-indexes
• status
• stats

Closes #2766

Changelog Entry

• Fixed: WP-CLI index command without the --network-wide only syncs the main site.
• Fixed: WP-CLI commands get-mapping, get-indexes, status, and stats only uses all sites' indices name when network activated.

Credits

Props @felipeelia @colegeissinger

[tott]

This looks good to me