Specify blob account settings at Repository creation time

[smflorentino]

Currently, when creating an Azure Repository for ES, creation fails if the proper blob storage settings aren't in elasticsearch.yml. Changing the storage account settings can be cumbersome if there is a large number of nodes, as it requires a full cluster restart.

I want to do a one-time back up some repositories for testing, but I don't have access to do a full reboot of the ES cluster. I'd like to be able to specify the blob storage account details right in the request (the AWS plugin supports this). I'm willing to do the work on this if it would be received positively.

[dadoonet]

I opened elastic/elasticsearch#12446 as this plugin will now be maintained there.

Thanks