Automatic merge from submit-queue (batch tested with PRs 51105, 51097, 51110, 50843, 51107)
gluster: Use VolumeHost.GetExec() to execute stuff in volume plugins
**What this PR does / why we need it**:
This PR updates GlusterFS volume plugin to use `VolumeHost.GetExec()` to execute utilities like mkfs and lsblk instead of simple `os/exec`. This prepares the volume plugin to run these utilities in containers instead of running them on the host + makes the volume plugin more independent and less hardcoded.
See proposal in https://github.com/kubernetes/community/pull/589.
Note that this PR does **not** change place where the utilities are executed - `VolumeHost.GetExec()` still leads directly to `os/exec`. It will be changed when the aforementioned proposal is merged and implemented.
**Special notes for your reviewer**:
There are two commits:
* The first one removes unused `plugin.execCommand` (so we don't need to update it to `VolumeHost.GetExec`)
* The second one does the `VolumeHost.GetExec` trick described above.
@kubernetes/sig-storage-pr-reviews
**Release note**:
```release-note
NONE
```
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