vault: allow overriding implicit vault constraint

[schmichael]

There's a bug in version parsing that breaks this constraint when using
a prerelease enterprise version of Vault (eg 1.3.0-beta1+ent). While
this does not fix the underlying bug it does provide a workaround for
future issues related to the implicit constraint. Like the implicit
Connect constraint: all implicit constraints should be overridable to
allow users to workaround bugs or other factors should the need arise.

Tested locally with Vault 1.3.0-beta1+ent and:

job "hello" {
  datacenters = ["dc1"]

  group "echo" {
                constraint {
                        attribute = "${attr.vault.version}"
                        operator  = "is_set"
                    }

    task "echo" {

      driver = "raw_exec"
      config {
        command = "sleep"
        args = ["10000"]
      }

      vault {
        policies    = ["default"]
        change_mode = "restart"
      }

      resources {
        cpu    = 100
        memory = 100
        network {
          mbits = 10
        }
      }
    }
  }
}

[github-actions]

I'm going to lock this pull request because it has been closed for 120 days ⏳. This helps our maintainers find and focus on the active contributions.
If you have found a problem that seems related to this change, please open a new issue and complete the issue template so we can capture all the details necessary to investigate further.