add spread feature

[jagedn]

this is a WIP implementation for spread using a "map" syntax, i.e.

jobs {
        spreads = {
            spread = name:'node.datacenter', weight: 50, targets: ['a': 20]
        }
    }

Attached you can see how the spread is sent to cluster in the Job defintion

[jagedn]

attached and example with the -profile localnomad so process directives is used also

[abhi18av]

Hi @jagedn ,

Thank you for taking this forward! I found a couple of neat design tricks in the code. Though I do confess that at the moment, I am unable to really find a use-case within the sun-nomadlab directly and therefore what validations exactly we need to add.

At some point, we need the DSL to dictate the legal/illegal values even if the underlying nomad layer allows a specific ability. I'm just not sure what aspect we'd need to put some gates upon.

Overall, to me this looks good to merge for now and we can continue with the v0.2.1 release so that @matthdsm and @tomiles can test this further.