Commits on Feb 10, 2020

1. client: enable configuring enable_tag_override for services …

   Consul provides a feature of Service Definitions where the tags
   associated with a service can be modified through the Catalog API,
   overriding the value(s) configured in the agent's service configuration.
   
   To enable this feature, the flag enable_tag_override must be configured
   in the service definition.
   
   Previously, Nomad did not allow configuring this flag, and thus the default
   value of false was used. Now, it is configurable.
   
   Because Nomad itself acts as a state machine around the the service definitions
   of the tasks it manages, it's worth describing what happens when this feature
   is enabled and why.
   
   Consider the basic case where there is no Nomad, and your service is provided
   to consul as a boring JSON file. The ultimate source of truth for the definition
   of that service is the file, and is stored in the agent. Later, Consul performs
   "anti-entropy" which synchronizes the Catalog (stored only the leaders). Then
   with enable_tag_override=true, the tags field is available for "external"
   modification through the Catalog API (rather than directly configuring the
   service definition file, or using the Agent API). The important observation
   is that if the service definition ever changes (i.e. the file is changed &
   config reloaded OR the Agent API is used to modify the service), those
   "external" tag values are thrown away, and the new service definition is
   once again the source of truth.
   
   In the Nomad case, Nomad itself is the source of truth over the Agent in
   the same way the JSON file was the source of truth in the example above.
   That means any time Nomad sets a new service definition, any externally
   configured tags are going to be replaced. When does this happen? Only on
   major lifecycle events, for example when a task is modified because of an
   updated job spec from the 'nomad job run <existing>' command. Otherwise,
   Nomad's periodic re-sync's with Consul will now no longer try to restore
   the externally modified tag values (as long as enable_tag_override=true).
   
   Fixes #2057

   

   shoenig committed Feb 10, 2020
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