make project Rakefile optional; don't shell out to rake

The Rakefile is now optional in all Omnibus projects that are built
using the `omnibus` CLI.

The `omnibus build` command now looks for an optional Omnibus
configuration file at the following locations:

* specified by the user using the `-c|--config` CLI option
* `$PWD/omnibus.rb`

If no valid configuration is found the build is run with the default
configuration (loaded by a call to `Omnibus.configure`). This will allow
a user to do things like have a system-wide Omnibus configuration file
which contains

The default project build Rake task is now invoked directly in the CLI
code which means no more shelling out to run Rake!
Don't CLOBBER install_dir which is hard coded to /opt/chef!

Do CLOBBER package_dir.