Allow ES instances to upgrade to gp3 volumes

[markdboyd]

Related to #339

Changes proposed in this pull request:

• Add new volume_type option when updating Elasticsearch instances
• Add validator for volume_type to only allow gp3 as a value
• Add logic to set EBS volume type (if changed) when updating an Elasticsearch instance
• Add new unit tests to cover new behavior

Things to check

• For any logging statements, is there any chance that they could be logging sensitive data?
• Are log statements using a logging library with a logging level set? Setting a logging level means that log statements "below" that level will not be written to the output. For example, if the logging level is set to INFO and debugging statements are written with log.debug or similar, then they won't be written to the otput, which can prevent unintentional leaks of sensitive data.

Security considerations

There are no other expected changes in functionality besides allowing users to set the volume_type to gp3 when running cf update-service on their Elasticsearch instances