Clean up gitkeep files, make hidden

The one in amazon/bin was checked into git as a symbolic link that would
be broken on other users' systems ;-)  Presumably happened before the
`find` command in awsenv-rehash excluded it.
Avoid settings vars for missing files, unset in that case

If files are missing, don't export vars with bogus values. You may not,
for instance, need to use EC2 for all of your AWS accounts, and
additionally EC2_CERT and EC2_PRIVATE_KEY and their associated options
for the EC2 CLI tools are deprecated.

If these variables indeed will not be exported, unset them so you can
switch between accounts without tainting the environment with mixed
account settings.
Only warn on validation of missing non-essential files

Instead of exiting with an error code.
Merge pull request #13 from ches/unset-vars-for-missing-elements

Unset vars for missing elements