COOPR-621 Do not require Internet for chef-solo bootstrap

Here's the flow:

yum -> apt -> curl -> wget -> FAIL

This should allow organizations to provide the `chef` package via their own YUM/APT repositories, which may reside inside their firewall.
suggestion: @ssh_file is an instance var but used like a local var in each of the 3 methods.  Also looking at it, I don't think we need it at all, as the File.delete where it's used can just check @ssh_keyfile directly.