Setting distinct_hosts to a boolean causes panic

[F21]

The document states that the distinct_hosts attribute of the constraints object should be a boolean:

# Define a job called my-service
job "consul" {
    # Job should run in the US region
    region = "global"

    # Spread tasks between us-west-1 and us-east-1
    datacenters = ["dc1"]

    # run this job globally
    type = "system"

    # Rolling updates should be sequential
    update {
        stagger = "30s"
        max_parallel = 1
    }

    constraint{
        distinct_hosts = true
    }

    group "consul-server" {
        ...
    }
}

However, if I set it to true, nomad panics:

panic: interface conversion: interface is bool, not string

goroutine 1 [running]:
github.com/hashicorp/nomad/jobspec.parseConstraints(0xc82000cae8, 0xc8201975a0, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/jobspec/parse.go:321 +0x757
github.com/hashicorp/nomad/jobspec.parseJob(0xc82000ca80, 0xc8200f9080, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/jobspec/parse.go:118 +0x502
github.com/hashicorp/nomad/jobspec.Parse(0x7f1169afc270, 0xc820026220, 0xc820026220, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/jobspec/parse.go:53 +0x404
github.com/hashicorp/nomad/jobspec.ParseFile(0xc8200f8940, 0x1a, 0x0, 0x0, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/jobspec/parse.go:73 +0x15c
github.com/hashicorp/nomad/command.(*RunCommand).Run(0xc8200f8900, 0xc82000a1b0, 0x1, 0x1, 0xc820106ea0)
        /opt/gopath/src/github.com/hashicorp/nomad/command/run.go:77 +0x28a
github.com/mitchellh/cli.(*CLI).Run(0xc8200b7c30, 0xc8200b7c30, 0x0, 0x0)
        /opt/gopath/src/github.com/mitchellh/cli/cli.go:112 +0x655
main.RunCustom(0xc82000a190, 0x3, 0x3, 0xc82015a390, 0x0)
        /opt/gopath/src/github.com/hashicorp/nomad/main.go:47 +0x469
main.Run(0xc82000a190, 0x3, 0x3, 0xfa48e8)
        /opt/gopath/src/github.com/hashicorp/nomad/main.go:15 +0x4c
main.main()
        /opt/gopath/src/github.com/hashicorp/nomad/main.go:11 +0x60

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
        /usr/local/go/src/runtime/asm_amd64.s:1696 +0x1

goroutine 5 [syscall]:
os/signal.loop()
        /usr/local/go/src/os/signal/signal_unix.go:22 +0x18
created by os/signal.init.1
        /usr/local/go/src/os/signal/signal_unix.go:28 +0x37

goroutine 8 [runnable]:
text/template/parse.lexText(0xc820083180, 0xf98208)
        /usr/local/go/src/text/template/parse/lex.go:237 +0x37e
text/template/parse.(*lexer).run(0xc820083180)
        /usr/local/go/src/text/template/parse/lex.go:206 +0x52
created by text/template/parse.lex
        /usr/local/go/src/text/template/parse/lex.go:199 +0x15d

goroutine 9 [runnable]:
github.com/hashicorp/nomad/command.(*Meta).FlagSet.func1(0xc820083280, 0xc8200f8900)
        /opt/gopath/src/github.com/hashicorp/nomad/command/meta.go:58
created by github.com/hashicorp/nomad/command.(*Meta).FlagSet
        /opt/gopath/src/github.com/hashicorp/nomad/command/meta.go:62 +0x3ce

[diptanu]

@F21 It looks like we are assuming that the bool value true/false for this constraint is in quotes. so distinct_hosts="true" works.

cc/ @dadgar @cbednarski Thoughts about the UX? I think the value that we get from the hcl parser is of type interface{} It might be nice for us to not treat the value as a string. I can have a PR going for this.

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