As discussed at https://github.com/elastic/elasticsearch-cloud-azure/issues/91#issuecomment-229113595, we know that the current `discovery-azure` plugin only works with Azure Classic VMs / Services (which is somehow Legacy now).
The proposal here is to rename `discovery-azure` to `discovery-azure-classic` in case some users are using it.
And deprecate it for 5.0.
Closes#19144.
The discovery-plugin has been broken since 2.x because the code was not compliant with the security manager and because plugins have been refactored.
closes#18637, #15630
Until now we had a cloud-azure plugin which is providing 3 distinct features:
* discovery on Azure
* snapshot/restore on Aure
* SMB store
This commit splits the plugin by feature so people can use either one or the other or both features.
Doc is updated accordingly.
David Pilato
Clinton Gormley
Tanguy Leroux
Jim Ferenczi