disconnect remote-exec immediately

[jbardin]

The remote-exec provisioner was not disconnecting immediately after provisioning. The remote exec context was tied to the lifetime of the provisioner, but the provisioner node used to not live long after the remote execution had completed so it was not noticed. Some more recent refactoring extends the lifetime of the node, and causes the provisioner connections to hang around longer when they're not needed.

Ensure we disconnect immediately after the remote execution is complete.

Fixes #34137

[apparentlymart]

(The Code Consistency Checks failure here was caused by my merge of #34806 which has made the Consul backend no longer depend on legacy/helper/schema. I expect that if you rebase this branch and run make syncdeps then that error should go away. If not, please let me know and I'll try to debug further!)

[github-actions]

Reminder for the merging maintainer: if this is a user-visible change, please update the changelog on the appropriate release branch.

[github-actions]

I'm going to lock this pull request because it has been closed for 30 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.