Metadata server: Use 301 redirect rather than 308

[stevenjm]

In #121, requests for the security-credentials URL were changed to redirect to security-credentials/ with a trailing slash. However, this uses http.StatusPermanentRedirect, which corresponds to a 308, whereas the real metadata API issues a 301 redirect (http.StatusMovedPermanently).

Some clients do not handle the 308 correctly and fail to retrieve credentials. We've noticed broken behaviour in the Go sdk version 1.4.10, which is used by docker-machine.

I will be raising a pull request to fix this shortly.