Configurable host lifetimes

Reconfigure running VM's lifetime with `curl` for fun and profit!

```
# Give 'o41xtodlvnvu5cw' a lifetime of 2 hours
$ curl -X PUT -d '{"lifetime":"2"}' --url vmpooler.company.com/vm/o41xtodlvnvu5cw
```

Not that this is not ADDITIONAL hours, it is setting the overall lifetime of the VM since checkout.
For the record the `case`/`when` block is to support future modification options, and catch invalid ones.