Add BlockQueryWaitTime Nomad Configuration Option

[jeffwecan]

This PR adds a BlockQueryWaitTime config file option / -nomad-block-query-wait-time CLI flag to be used when folks want to either:

1. have the autoscaler's Nomad API client wait longer than five minutes for blocking query requests to return a response
2. or have the client wait a shorter duration for such responses (for instance, if there is a LB idle timeout < 5 minutes in between the autoscaler and associated Nomad API)

I.e., Resolves #754

---

Aside from the test case updates included as part of this PR's changes, a bit of practical testing was performed as well. Specifically with a shorter-than-5m-default wait time, but still longer than our 60 LB timeout, configured for the autoscaler:

nomad {
 namespace = "<our_namespace>"
 block_query_wait_time = "2m"
}

Combined with our dev deployment's 60 second idle timeout Nomad LB, we hit the precipitating error but now with our non-default 2m / 120000ms wait time reflected (yay):

failed to call the Nomad list policies API: Get "https://<our_nomad>:4646/v1/scaling/policies?index=47390046&namespace=<our_namespace>&wait=120000ms": EOF

Subsequently, we set block_query_wait_time = "56s" which was seen to prevent such errors with our particular Nomad API connectivity arrangements.

---

Also going to link the initial PR adding blocking queries here (for general context + so there is a breadcrumb to this PR in that preceding one): #38

[jeffwecan]

As a general note regarding the option name choice: I waffled between a few options before settling on this. Primarily motivated by this bit of similar config in the Nomad codebase (though related to a Consul API client setting in that case 🤷🏻): https://github.com/hashicorp/nomad/blob/v1.7.0-beta.1/client/config/config.go#L380-L387

(Happy to call it whatever though 😄 )

[lgfa29]

I think the name looks good to me 👍

[jeffwecan]

@jrasell: Would it be possible to get this PR reviewed ahead of the next autoscaler release? 🙏🏻

[lgfa29]

Thank you for the PR @jeffwecan!

And apologies for the delay in getting this merged. Nomad 1.7 and Autoscaler HA kept us busy for a while. I'm going over the opened PRs and issues to prepare a new release.

[lgfa29]

I think the name looks good to me 👍