Disable chef-solo run on slaves as a temporary quick fix to unblock Mac builds.

Can be reverted when https://github.com/opscode-cookbooks/omnibus/pull/12 is merged and released.

Note that currently this functionality is not needed since we are far away from rebuilding slaves using cookbooks in ci.opscode.us.
Merge pull request #117 from opscode/mac-build-fix

Disable chef-solo run on slaves as a temporary quick fix to unblock Mac builds.