Add ssd storage class for review apps

What does this MR do?

The ssd storage class is configured for Gitaly pods so that Gitaly has better IO performance.

The storage class was added to the cluster manually and is defined as:

apiVersion: storage.k8s.io/v1
kind: StorageClass
metadata:
  name: ssd
parameters:
  type: pd-ssd
provisioner: kubernetes.io/gce-pd

See #334342 (closed)

Does this MR meet the acceptance criteria?

Conformity

• [-] I have included changelog trailers, or none are needed. (Does this MR need a changelog?)
• [-] I have added/updated documentation, or it's not needed. (Is documentation required?)
• [-] I have properly separated EE content from FOSS, or this MR is FOSS only. (Where should EE code go?)
• [-] I have added information for database reviewers in the MR description, or it's not needed. (Does this MR have database related changes?)
• I have self-reviewed this MR per code review guidelines.
• This MR does not harm performance, or I have asked a reviewer to help assess the performance impact. (Merge request performance guidelines)
• I have followed the style guides.
• [-] This change is backwards compatible across updates, or this does not apply.