Allow Nomad to set Vault default lease duration for non-leased credentials (KVv2)

### Proposal
Nomad should be allowed to set the Vault default lease duration for non-leased credentials. This is already exposed in consul-template as vault.default_lease_duration.

### Use-cases
In cases where allocations either have a large number of secrets this can cause load issues on Vault, or a very large number of allocations monitoring a few secrets that are rarely written a 5 minute re-check duration does not make much sense.

### Attempted Solutions
Set `ttl` keys (works with kvv1, for better or worse), explore all kinds of places where this could be set, but isn't.

Current solution is using [[[ ]]] delimiters with `template{}` and running an actual `consul-template` sidecar, which is strictly unnecessary and a lot of added complexity.

Start by tracing Nomad's Vault template integration and compare it with the consul-template setting named in the issue, vault.default_lease_duration. Check how KVv2 non-leased credentials are rechecked and identify the configuration entry point; done means Nomad users can set that duration without running a consul-template sidecar.

Written by the indexing model from the issue text.