Bootstrap script should allow alternate destinations for binaries and user-data

**Describe the solution you'd like**
The bootstrap script is hardcoded to install binaries into `/usr/local/bin`, and to download user-data into `/etc/user-data`. This prevents running this script without root permissions, even though none of these artifacts _need_ to be installed system-wide.

**Describe alternatives you've considered**
- Manually install the prerequisites (`fetch` and `gruntwork-installer`), but this is error-prone.
- Modify the bootstrap script to use alternate directories (I have done this, PR incoming).

**Additional context**
This is a minor change that can be accomplished by overriding the default values for two environment variables. It may not be widely used, and would be easily discovered by advanced users upon inspection of the script, so it does not need associated CLI args or even documentation.

Locate the bootstrap script and inspect the hardcoded /usr/local/bin and /etc/user-data destinations. Add the two environment-variable overrides described in the issue, then verify the script can use alternate paths without root; no test file is named in the issue.

Written by the indexing model from the issue text.