Add Levant dispatch command to dispatch against Nomad jobs.

[jrasell]

Levant now has the functionality to dispatch against Nomad jobs
using the dispatch command. Jobs that are dispatched go through
the job status checker and so dispatched jobs can be confirm to
reach the running state.

Closes #113

[jrasell]

@sevagh I had some time to complete my started work today so raised this PR and would love any feedback you have.

[sevagh]

I'll compare this directly to my code later today, but it looks good. Mine is definitely more hacky (e.g. not supporting payloads, and passing -meta as a single comma-separated list instead of multiple flags).

I suspect the dispatch behavior is the same as nomad job dispatch? Return a successful allocation.

I'll compile your branch and run it with my arguments today as a sanity check.

[jrasell]

The run behaviour is identical; but it will go through the additional checking that Levant provides and so will ensure that the dispatched job reaches the running state as well as all the allocations that are part of it.

[stevenscg]

I use the heck out of dispatch jobs. Nice work!

[sevagh]

Works nice for me.

I still rely on code from here (-monitor and -timeout) to poll Nomad and check for the end result of the dispatched job.

[sevagh]

Once that other PR is merged independently, extending both deploy and dispatch to support the blocking behavior should be entirely possible. I didn't mean to imply that this PR is unusable without the other one.