Selectively load softwares and projects

Fixes #203 

Preliminary benchmarks:

```text
# Current
$ bundle exec omnibus build hamlet -l debug
0.97s user 0.43s system 69% cpu 2.008 total

# With patch
$ bundle exec omnibus build hamlet -l debug
0.90s user 0.33s system 73% cpu 1.684 total
```

It seems we are consistently shaving ~0.5s. However, when there are a lot of software and project definitions (omnibus-chef):

```text
# Current
$ bundle exec omnibus list
3.21s user 0.75s system 88% cpu 4.455 total

# With patch
$ bundle exec omnibus list
0.84s user 0.31s system 75% cpu 1.537 total
```

In this instance, we can see a very significant gain in speed.

/cc @lamont-granquist @danielsdeleo @opscode/release-engineers