[hab] Add a simple feature toggle to use a static Supervisor.
[hab] Add a simple feature toggle to use a static Supervisor.

This change introduces a simple feature toggle which allows the `hab`
binary to use a statically build version of the Supervisor rather than
the current glibc dynamically linked version.

As this is the first feature toggle that I know of in the codebase,
we'll start really simple with an environment variable implementation
and think carefully about scaling up to more :)

To back the `hab start` and `hab sup` subcommands with the
`core/hab-sup-static` pacakge rather than the default `core/hab-sup`
package, set the `HAB_FEAT_SUP_STATIC` environment variable to a
non-empty value. Otherwise our default behavior continues as normal,
using the `core/hab-sup` package.

For example:

```
> env RUST_LOG=info,hab::command::sup=debug,hab::exec=debug HAB_FEAT_SUP_STATIC=true hab start core/redis
DEBUG:hab::command::sup: Enabling statically compiled Supervisor from core/hab-sup-static
DEBUG:hab::exec: Calling execv: ("/hab/pkgs/core/hab-sup-static/0.6.0/20160602141707/bin/hab-sup") ["start", "core/redis"]
hab-sup(MN): Starting core/redis
hab-sup(GS): Supervisor 172.17.0.2: 13d12e83-de28-4ff9-8674-a64af032952b
hab-sup(GS): Census redis.default: abf0c60c-1cae-40bc-b1a3-fc64d96a5193
hab-sup(GS): Starting inbound gossip listener
-- snip --
```

Signed-off-by: Fletcher Nichol <fnichol@nichol.ca>

Pull request: #625
Approved by: juliandunn

georgemarshall/habitat,georgemarshall/habitat,habitat-sh/habitat,habitat-sh/habitat,habitat-sh/habitat,rsertelon/habitat,rsertelon/habitat,georgemarshall/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,nathenharvey/habitat,habitat-sh/habitat,rsertelon/habitat,habitat-sh/habitat,habitat-sh/habitat,georgemarshall/habitat,nathenharvey/habitat,nathenharvey/habitat,georgemarshall/habitat,georgemarshall/habitat,georgemarshall/habitat,nathenharvey/habitat,rsertelon/habitat,nathenharvey/habitat,georgemarshall/habitat,habitat-sh/habitat,rsertelon/habitat