release-23.1: roachtest: stop using ClusterSpec.Cloud in test code

[RaduBerinde]

Backport 5/5 commits from #111324.

/cc @cockroachdb/release

Release justification: test-only change, keeping roachtest in sync

---

In this PR we fix all test registration code which was depending on the flags passed to roachtest (indirectly, through TestSpec). In the process, I tried to clean up various things I had to touch.

This makes progress in the direction of #104029 and of making the registry and TestSpec not depend at all on the flags (the flags should come in later, when we create an actual cluster from the spec).

roachtest: spec: simplify Option

This commit reduces boilerplate in the Option code by making it a
func instead of an interface with an apply func. This way each option
can define the function inline instead of having to define a type.

Epic: none
Release note: None

roachtest: spec: provide options for GCE/AWS settings

This commit moves GCE and AWS specific settings to their own
inline structs and adds Options for them.

Epic: none
Release note: None

roachtest: clean up instance type specification

ClusterSpec now provides options for GCE and AWS machine types.

Epic: none
Release note: None

roachtest: clean up zones specification

ClusterSpec now provides options for GCE and AWS zones
specification.

Epic: none
Release note: None

roachtest: stop using ClusterSpec.Cloud in test code

This change removes all remaining uses of ClusterSpec.Cloud except
those internal to roachtest. Code that is part of running a test now
uses Cluster.Cloud() instead.

Informs: #104029
Release note: None

[blathers-crl]

Thanks for opening a backport.

Please check the backport criteria before merging:

• Backports should only be created for serious
  issues or test-only changes.
• Backports should not break backwards-compatibility.
• Backports should change as little code as possible.
• Backports should not change on-disk formats or node communication protocols.
• Backports should not add new functionality (except as defined
  here).
• Backports must not add, edit, or otherwise modify cluster versions; or add version gates.
• All backports must be reviewed by the owning areas TL and one additional
  TL. For more information as to how that review should be conducted, please consult the backport
  policy.

If your backport adds new functionality, please ensure that the following additional criteria are satisfied:

• There is a high priority need for the functionality that cannot wait until the next release and is difficult to address in another way.
• The new functionality is additive-only and only runs for clusters which have specifically “opted in” to it (e.g. by a cluster setting).
• New code is protected by a conditional check that is trivial to verify and ensures that it only runs for opt-in clusters. State changes must be further protected such that nodes running old binaries will not be negatively impacted by the new state (with a mixed version test added).
• The PM and TL on the team that owns the changed code have signed off that the change obeys the above rules.
• Your backport must be accompanied by a post to the appropriate Slack
  channel (#db-backports-point-releases or #db-backports-XX-X-release) for awareness and discussion.

Also, please add a brief release justification to the body of your PR to justify this
backport.

[cockroach-teamcity]

This change is 

[RaduBerinde]

https://teamcity.cockroachdb.com/viewQueued.html?itemId=12969525

[RaduBerinde]

TFTR!