HELP WANTED: availableWorkers()

# Background
When submitting a job to the TORQUE / PBS using something like:
```sh
qsub -l nodes=3:ppn=2 myjob.sh
```
the scheduler will allocate 3 nodes with 2 cores each (= 6 cores total) for `myjob.sh` when launched. Exactly which 3 nodes is only known to `myjob.sh` at run time. This information is available in a file `$PBS_NODEFILE` written by TORQUE / PBS, e.g.
```sh
$ cat $PBS_NODEFILE
n1
n1
n8
n8
n9
n9
```
Other HPC job schedulers use other files / environment variables for this.

# Actions
Add an `availableNodes()` file that searches for common environment variables and returns a vector of node names, e.g.
```r
> availableNodes()`
[1] "n1" "n1" "n8" "n8" "n9" "n9"
```
If no known environment variables are found, the default fallback could be to return `rep("localhost", times = availableCores()`.

The above would allow us to make `workers = availableNodes()` the new default for `cluster` futures (currently `workers = availableCores()`).

Identify these settings for the following schedulers:
- [x] [PBS](https://en.wikipedia.org/wiki/Portable_Batch_System) (Portable Batch System): Environment variable `PBS_NODEFILE` (the name of a file containing one node per line where each node is repeated "ppn" times).
- [x] [Oracle Grid Engine](https://en.wikipedia.org/wiki/Oracle_Grid_Engine) (aka Sun Grid Engine, CODINE, GRD). Environment variable `PE_HOSTFILE` (a file, format unclear), cf. https://www.ace-net.ca/wiki/Sun_Grid_Engine
- [x] [Slurm](https://en.wikipedia.org/wiki/Slurm_Workload_Manager) (Simple Linux Utility for Resource Management). Environment variable `SLURM_JOB_NODELIST` (list of nodes in a compressed format, e.g. instead of "tux1,tux3,tux4" it is stored as "tux[1,3-4]". Note that multiple "compressions" may exist, e.g. ["compute-[0-6]-[0-15]"](https://groups.google.com/forum/#!topic/slurm-devel/n6x2WgGmDls). The _number_ of nodes is can be verified by `SLURM_JOB_NUM_NODES`. The "ppn" information is in stored in `SLURM_TASKS_PER_NODE`).
- [x] LSF/[OpenLava](https://en.wikipedia.org/wiki/OpenLava) (Platform Load Sharing Facility).
- [x] `LSB_HOSTS`
- [ ] [Spark](https://spark.apache.org/)
- [ ] [OAR](http://oar.imag.fr)
- [ ] [HTCondor](https://en.wikipedia.org/wiki/HTCondor)
- [ ] [Moab](https://en.wikipedia.org/wiki/Moab_Cluster_Suite)
- [x] PJM (https://staff.cs.manchester.ac.uk/~fumie/internal/Job_Operation_Software_en.pdf)
- [x] `PJM_O_NODEINF` - "Path of the allocated node list file. For a job to which virtual nodes are allocated, the IP addresses of the nodes where the virtual nodes are placed are written one per line."