In order to move multi perspective validation forward we need to support policy
in Boulder configuration that can relax multi-va requirements temporarily.
A similar mechanism was used in support of the gradual deprecation of the
TLS-SNI-01 challenge type and with the introduction of CAA enforcement and has
shown to be a helpful tool to have available when introducing changes that are
expected to break sites.
When the VA "multiVAPolicyFile" is specified it is assumed to be a YAML file
containing two lists:
1. disabledNames - a list of domain names that are exempt from multi VA
enforcement.
2. disabledAccounts - a list of account IDs that are exempt from multi VA
enforcement.
When a hostname or account ID is added to the policy we'll begin communication
with the related ACME account contact to establish that this is a temporary
measure and the root problem will need to be addressed before an eventual
cut-off date.
Resolves https://github.com/letsencrypt/boulder/issues/4455
ddfc620c44