Fix error parsing config when setting consul.timeout.

[jrasell]

When parsing a config file which had the consul.timeout param set,
Nomad was reporting an error causing startup to fail. This seems
to be caused by the HCL decoder interpreting the timeout type as
an int rather than a string. This is caused by the struct
TimeoutHCL param having a hcl key of timeout alongside a Timeout
struct param of type time.Duration (int). Ensuring the decoder
ignores the Timeout struct param ensure the decoder runs
correctly.

closes #6893

[tgross]

LGTM. Thanks @jrasell!

It looks like this got introduced in ae7a8da and while there are some other examples of this config parsing w/ time.Duration in that commit, this is the only one where we have this kind of accidental shadowing happen.

[github-actions]

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