storage/kubernetes: don't guess the kubeconfig location and change test env

Using the default KUBECONFIG environment variable to indicate that
the Kubernetes tests should be run lead to cases where developers
accidentally ran the tests. This has now been changed to
"DEX_KUBECONFIG" and documentation hsa been added detailing how to
run these tests.

Additionally, no other storage reads environment variables for its
normal configuration (outside of tests) so the Kubernetes storage
no longer does.

Overall, be less surprising.

contrib/openldap: add an OpenLDAP Docker image for testing

*: port oob template

Documentation/proposals: add a proposal for keeping data in-sync during refreshes

server: fix key rotation polling

gorename command run:

    gorename \
      -from '"github.com/coreos/dex/server".rotationStrategy.period' \
      -to rotationFrequency

*: build aci at the correct path including version, OS, and arch

dev branch: fix sql keys query

*: add more comments to the example config

dev branch: self managed third party resources

server: only print gc stats if something has been removed

dev branch: fix linting

dev branch: properly compile version into docker images

dev branch: add DCO and LICENSE

dev branch: add garbage collect method to storage