Make Segment Store cache volume optional for Pravega +0.7

[RaulGracia]

Description

In Pravega 0.7, the Segment Store cache is completely in-memory. This means that the PVC that the operator creates is not necessary anymore. We need to make the deployment of this volume optional, depending on whether the version of Pravega is older or newer than 0.7.

Importance

Must have for 0.7.

Location

Segment Store PVC creation.

Suggestions for an improvement

We need to consider whether if we want to make this as a flag in the Operator manifest or whether the Operator should automatically look at the Pravega version and deploy the volume or not based on that.