Allow for caching the omnibus download via -d option to install.sh

This PR allows you to control the download directory of the omnibus package via the `-d` option of the `install.sh` (see related PR opscode/opscode-omnitruck#33) via the `OMNIBUS_DOWNLOAD_DIR` env var.

If not specified the install.sh is called without the `-d` option, i.e. the package is downloaded to a temp directory.

If  `OMNIBUS_DOWNLOAD_DIR` is specified it will be passed to install.sh, which effectively allows for caching the downloaded omnibus package (will be reused if exists and checksum matches rather than being downloaded again).