va: prepare to require minimum of 3 RVAs

[jsha]

To prepare for the MPIC requirement of having a minimum of 3 perspectives, I added code to NewValidationAuthorityImpl to error if there aren't enough remote VAs configured and the current VA is the primary perspective. Then I fixed all the tests, which involved adding some backends in the unittests, and spinning up remoteva-c in the integration tests.

As a reminder, the boulder va command always considers itself the primary perspective, while boulder remoteva gives itself a perspective based on its config.

I wound up backing out the code in NewValidationAuthorityImpl because right now our remote VAs are actually running the boulder va command, so they would error out in prod, even though our actual primary perspective does have enough backends. So this wound up as a test-only change.

[github-actions]

@jsha, this PR appears to contain configuration and/or SQL schema changes. Please ensure that a corresponding deployment ticket has been filed with the new values.

The base branch was changed.

[jsha]

One particular change to note with the merge: the Local VA ok, 1/2 remote VA internal err test case is removed (even though it was just added) because it no longer makes sense to have only 2 remote VAs. The primary VA would error at construction time.