Allow separate enterprise config overlay

Show an error message when forcing a periodic launch is forbidden

UI: Use native fetch in production when available

UI: The initial index qp value should be 1, not 0

Add an isProduction condition to ensure native fetch is used when
possible.

Drain v2: add controlled draining

Also switch to require and add t.Helper to appropriate funcs.

Node drainer would throw off the index checks

drainer was unsetting drain before fsm could read written value

Also drop the New func as it's easy to swap the order of arguments since
they're both strings.

Test job watcher

Chan must be buffered to avoid skipping triggering altogether

Also made timing in a test a bit more lenient

Also delay "node complete" after the node has been marked complete to
capture a few more alloc events. There are other ways to implement this
that could trade off correctness for responsiveness as technically a
node is considered drained when all of its allocs have been marked to
stop and not when they've actually stopped (which may not happen for a
long time).

allow -detach like other commands

This PR allows marking a node as eligible for scheduling while toggling
drain. By default the `nomad node drain -disable` commmand will mark it
as eligible but the drainer will maintain in-eligibility.