Fix key generation

This fixes eyaml key generation on Puppet open source, which would error out with messages like:
```
Error: Could not find command '/usr/bin/puppet/eyaml'
```
As the eyaml binary may be in one of several directories depending on both OS and which gem provider is in use, switch to using a search path, and make it configurable.

This should cover both #30 and #32, and also work on Ubuntu versions where gems end up in /usr/local/bin.