[FEATURES] Upgrade Alluxio docker image to version 2.9.4

[cheyang]

What feature you'd like to add:

Currently alluxioRuntime is using alluxio docker image version 2.9.0 : https://github.com/fluid-cloudnative/fluid/blob/master/charts/fluid/fluid/values.yaml#L69

We'd like to keep up the latest release version: https://github.com/Alluxio/alluxio/releases/tag/v2.9.4

We need do compatible test with the new alluxio version with the following test cases: https://github.com/fluid-cloudnative/community/blob/master/testcase/fluid-testcase.md

Why is this feature needed: