Support for vault namespaces

[cgbaker]

Updated Vault integration to add initial support for Vault namespaces.

Updates to:

• Vault configuration stanza (server and client)
• Vault client usage on server and client
• consul-template configuration
• numerous dependencies updates to support this
• documentation

Note, the circonus-gometrics dependency in vendors was updated to work with the updated go-retryablehttp dependency. This is relatively low-risk for future updates, because it corrected a compile-error; if some future hypothetical update of circonus-gometrics doesn't compile, it will be pretty noticeable. Also, the change was a trivial function signature change:
https://github.com/hashicorp/nomad/pull/5520/files#r272587624

There's a question as to whether VAULT_NAMESPACE should be included on the default environment blacklist

Also, this apparently broke the vault e2e test in Travis; an endpoint used in the test suite changed in the Vault API; new versions of Vault are backwards compatible with old clients, but the new vault.api client is not compatible with old Vault servers. This endpoint isn't used in Nomad's Vault integration; it's only present as part of setup in the Vault e2e test suite. This has been modified to bypass the client in favor of a raw request against the previous API when testing against an older version of Vault. (I also added the latest versions of Vault to the test matrix.)

[schmichael]

... circonus-gometrics ...

So the change are manually applied to our vendor? I think it'd be better to fork the repo to hashicorp, apply the update, use that in nomad, and submit a pr upstream. Looks like someone already reported this upstream, but there's no activity: circonus-labs/circonus-gometrics#94 (can be handled in a followup PR)

There's a question as to whether VAULT_NAMESPACE should be included on the default environment blacklist

Might this be necessary to include in task environments for services which use their Vault token to interact with Vault directly? (can be handled in a followup PR)

[cgbaker]

There's a question as to whether VAULT_NAMESPACE should be included on the default environment blacklist

Might this be necessary to include in task environments for services which use their Vault token to interact with Vault directly? (can be handled in a followup PR)

Implemented in the following:
#5556

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