Update fingerprint generation command

[zankich]

While following the AWS guide, we ran into issues with the fingerprint which was generated. The guide currently creates a SHA256 fingerprint, and this pull request changes the ssh-keygen command to generate an md5 fingerprint.

Without this change we were getting the following error message while attempting to SSH into an app deployed onto our CF installation.

○ → cf ssh dora -i 0
FAILED
Error opening SSH connection: ssh: handshake failed: Unsupported host key fingerprint format

As well as the following acceptance-tests failues

[Fail] {NO_DEA_SUPPORT} SSH ssh [It] can execute a remote command in the container
/Users/pivotal/go/src/github.com/cloudfoundry/cf-acceptance-tests/ssh/ssh_test.go:68

[Fail] {NO_DEA_SUPPORT} SSH ssh [It] runs an interactive session when no command is provided
/Users/pivotal/go/src/github.com/cloudfoundry/cf-acceptance-tests/ssh/ssh_test.go:105

[Fail] {NO_DEA_SUPPORT} SSH ssh [It] allows local port forwarding
/Users/pivotal/go/src/github.com/cloudfoundry/cf-acceptance-tests/ssh/ssh_test.go:126

cc @kkallday

[cf-gitbot]

We have created an issue in Pivotal Tracker to manage this. You can view the current status of your issue at: https://www.pivotaltracker.com/story/show/117855121.

[emalm]

Thanks, @zankich. This seems to be a behavioral change in ssh-keygen: on OS X 10.10 Yosemite and Ubuntu Trusty, ssh-keygen always produces the colon-separated MD5 hash and has no -E flag. On OS X 10.11 El Capitan and Ubuntu Xenial, it behaves as documented. Would you mind updating the documentation to cover both behavioral cases? I unfortunately don't think that there's a single command that works for both, so maybe present the -E-flag command as the primary case and the older, non--E case as the fallback?

Thanks again,
Eric, CF Runtime Diego PM

[zankich]

@ematpl we've updated the instructions for older OS version. Let us know if you'd like anything else changed!

[emalm]

Thanks, @zankich! I'll merge it once the diego-release can-i-bump light goes green.

Best,
Eric