Add a new optional `only_factor` parameter for the simulate_factors()
method.

The only_factor parameter is specified, only a single simulation will be
run. The simulation that runs is for the factor at the index specified
by the value of the only_factor argument.

For example, if the snowbird command-line arguments look like this:

	meta.cmd_line: sb -f host.queue_depth 4,16 -f fm.fim.xfer_rate_MBps 400,800 -f fm.ase.t_wait_rd 4,5,6 -s duration 10 ms --only-factor 4

Then snowbird will pass in only_factor=4 as the argument to desmod and
the meta.sim.index and meta.sim.workspace will be set to 4 and
workspace/4 respectively.

Additionally, meta.sim.special will look like this:

    meta.sim.special:
    - [host.queue_depth, 4]
    - [fm.fim.xfer_rate_MBps, 800]
    - [fm.ase.t_wait_rd, 5]

That is, only a single simulation, for the factor indicated by
meta.sim.special, was run.

One use for this new feature is orchestrating the execution of multiple,
per-factor, simulations on separate machines (e.g., in the cloud) with a
single command (with multiple factors).

Some new config options have been added:
 - sim.workspace.s3_sync: When true, syncs any workspaces to s3
 - sim.workspace.s3_bucket: The s3 bucket to sync to
 - sim.workspace.s3_sync_prefix: A string to act as the root path in the
   s3 bucket.  By default, this is set the user's username.

Here's an example of running snowbird with these new options:

$ sb -s sim.workspace ~/locker/test -s sim.workspace.s3_sync_prefix
bgmerrell -n test-rd-rnd -s sim.duration 10ms -s sim.workspace.s3_sync
True -f host.queue_depth 4,16

This would result in the following being synced to the default bucket in
s3:

/bgmerrell/test/0/config.yaml
/bgmerrell/test/0/result.yaml
/bgmerrell/test/0/sim.log
/bgmerrell/test/1/config.yaml
/bgmerrell/test/1/result.yaml
/bgmerrell/test/1/sim.log

The method to do the sync work is called when the _Workspace context
manager is exiting.

The uploading to s3 is done using the concurrent.features package.  This
package isn't available in the Python 2 library, so the requirements.txt
file has been updated to install the package for Python 2 only.

* only_factor -> config_filter
* use setdefault() for some workspace sync config keys
* default s3 sync prefix is now ""
* don't importa boto3 or concurrent.futures until needed.
* sim.workspace.s3_sync -> sim.sync.s3.enable (and similar for other s3
  sync-related keys)

The sync code will not live in desmod.  We've decided to take a
different approach.