Issues with fetch and PATH

`bootstrap-gruntwork-installer.sh` installs `fetch` into `/usr/local/bin`. Depending on the OS and the user (e.g., root), this directory is not always on the `PATH`. As a result, the call to `fetch` in `gruntwork-install` may fail.

We should either (1) call `fetch` from an absolute path in `gruntwork-install` or (2) install `fetch` into something more likely to be on the `PATH`, such as `/usr/bin`.

Inspect bootstrap-gruntwork-installer.sh and the fetch call in gruntwork-install, then check behavior when /usr/local/bin is absent from PATH, including for root. Confirm the chosen path-handling approach works across the affected OS and that gruntwork-install can invoke fetch successfully.

Written by the indexing model from the issue text.