vault: Relax backend instantiation configuration constraints

For the migration context case, it's ok to relax the constraints on the configuration not specifying the cache, scheduler or storage key entries (because they are not used).

It's not the case for the actual runtime case (which requires them). So assertions in the code using those backends has been added to fail as soon as possible.

[1] Successful migration check run with this patch

swh@vault-rpc-697776867c-mslf2:~$ swh db --config-file=$SWH_CONFIG_FILENAME version vault

module: vault:postgresql
current code version: 4
version: 4

[2] Success runtime (rpc) context run with this patch

swh@vault-rpc-697776867c-mslf2:~$ ./entrypoint.sh
Starting the swh-vault API server
Connection in use: ('0.0.0.0', 5005)
connection to ('0.0.0.0', 5005) failed: [Errno 98] Address already in use

Note: ^ failure to connect since the pod has already the main process attached to it (expectedly), run to entertain there is no failure regarding backend instantiation with the changes.

[3] Current failure with latest version (without the patch)

-> https://gitlab.softwareheritage.org/-/snippets/1648

Refs. swh/infra/sysadm-environment#5562 (closed)