implementation of possibility to add multiple volumes

[jagedn]

This implementation introduces a new config volumes (plural) as a List of VolumeSpec closures

Tested against the validation folder:

 jobs {
        volume = { type "host" name "scratchdir" }

        volumes = [
            { type "host" name "scratchdir" path "/var/data1" },
            { type "host" name "scratchdir" path "/var/data2" }
        ]
    }

• you can use volume , volumes or a mix of both
• you can/need to specify the path in "secondary" volumes
• you can specify which volume uses as workDir (so path is not required)

closes #45

[jagedn]

Looks good as far as I can understand the code :)

You can take a look to the examples in the validation folder, these are "real" configurations executed against a local nomad instance

https://github.com/nextflow-io/nf-nomad/pull/46/files#diff-ee3e4320a49d7523b0af681bbc50ede9992e4094046302a2682764cdf9b44624