Documentation: add a document on storage options

Merge pull request #626 from ericchiang/storage-kubernetes-guess-namespace-from-service-account-token

storage/kubernetes: guess namespace from the service account token

*: expand environment variables in config

storage/kubernetes: set CurrentContext when the Kubeconfig file contains only one context

storage/kubernetes: don't automatically print errors on bad HTTP status codes

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.

These status codes spam the error logs for events like key rotation
and third party resource creation. In these cases "bad" status codes
are expected and shouldn't be automatically printed.

Allow users to define config values which are read form environemnt
variables. Helpful for sensitive variables such as OAuth2 client IDs
or LDAP credentials.

The in cluster kubernetes client currently requires using the
downward API to determine its namespace. However this value can be
determine by inspecting the service account token mounted into the
pod. As a fallback, use this to guess the current namespace.

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