377 work dir config

Addresses #377 (move task work directory config back to suite definition [runtime] to allow shared  "current working directories")

To get the best of both of worlds, I've implemented this in the following form:

```
# site/user config
[hosts]
   [[localhost]] #etc. (other hosts)
          run directory = $HOME/cylc-run      # top level for all logs, run-db, etc.
          work directory = $HOME/cylc-run    # top level for suite share and task work dirs
```

and 
```
# suite.rc
[runtime]
    [[namespace]]
          work sub-directory = $CYLC_TASK_ID    # $HOME/cylc-run/<suite>/work/<ID>
```

So: we can still configure the top level work directory location for all suites in site/config; but the final task work sub-directory is configurable (if necessary) in suite [runtime].