roachtest: use pd-ssd as default, even when VolumeSize is set

[lidorcarmel]

The cluster_to_cluster roachtest was setting VolumeSize, and assuming that the default pd-ssd is used, but instead the roachtest was running with standard pd (HDD). If VolumeSize is not set then roachtest is using local ssd, and if it is set then it uses whatever is in the GCEVolumeType. normally roachtest users don't set GCEVolumeType, and therefore roachtest uses an empty string for the pd type (which again means HDD).

Instead, this pr keep the default volume type unless the user asked for something else.

Epic: CRDB-25146

Release note: None

[cockroach-teamcity]

This change is 

[msbutler]

nice catch lidor! Just noting that,

• this diff will affect the restore roachtests as well
• the roachprod docs do suggest that pd-ssd is the default:

roachprod create -h | grep 'gce-pd-volume-type'
      --gce-pd-volume-type string                                                Type of the persistent disk volume, only used if local-ssd=false (default "pd-ssd")

But before making this change, do we know what customers usually use? pd-ssd or standard pd?

[msbutler]

ah, well actually, this might have been a regression introduced a few weeks ago in #103757

[lidorcarmel]

do we know what customers usually use? pd-ssd or standard pd?

pd-ssd.

this might have been a regression introduced a few weeks ago in #103757

can you please explain?

[msbutler]

the above PR added this line:
https://github.com/cockroachdb/cockroach/blob/master/pkg/cmd/roachtest/spec/cluster_spec.go#L186

[lidorcarmel]

Thanks!
bors r+ single on

[healthy-pod]

bors r-

[craig]

Canceled.

[healthy-pod]

bors r+

[healthy-pod]

bors single off

[craig]

Build failed:

• Bazel Essential CI (Cockroach)

[lidorcarmel]

bors r+

[craig]

Build failed (retrying...):

• Bazel Essential CI (Cockroach)

[craig]

Build succeeded:

• Bazel Essential CI (Cockroach)