Fix panic when using invalid JSON jobspec

[lgfa29]

Currently, when the input jobspec is in JSON format, but the content doesn't follow the expected Nomad JSON structure, the provider will panic when trying to call the Nomad API.

This PR changes the jobpsec parsing logic to gracefully fail when the in this situation.

It also allows for the JSON input to have a root element so that the provider can handle outputs from the Nomad CLI directly:

$ nomad job run -output jobspec.hcl
{
    "Job": {
        "Stop": null,
        "Region": "us",
...
}

[cgbaker]

LGTM