Update modules for go 1.11.4+

Go 1.11.3 changed how checksums are created in some cases, which caused
failures building via modules. (ref golang/go#29278)

Update the checksums for the failing modules.

To catch this is the future, a modules build was added to the build matrix. I
also noted that we were pinning the `.0` patchlevel of each go version which
wouldn't have picked this up, updated it to build with the latest patch
release.

Use github login as the id

server: update health check endpoint to query storage periodically

Instead of querying the storage every time a health check is performed
query it periodically and save the result.

Display access token in example app

ldap_test: add filter tests

Add Active Directory and kubelogin integration sample.

The filters for user and group searches hadn't been included in our LDAP
tests. Now they are.

The concrete test cases are somewhat contrived, but that shouldn't
matter too much. Also note that the example queries I've used are not
supported in AD: https://stackoverflow.com/a/10043452Signed-off-by: Stephan Renatus <srenatus@chef.io>

For downstream apps using a github handle is much simpler than working
with numbers.

WHilst the number is stable and the handle is not - GitHUb does give you
a big scary wanring if you try and change it that bad things may happen
to you, and generally few users ever change it.

This can be enabled with a configuration option `useLoginAsId`

update MAINTAINERS, mirror guidelines into dev doc

Bump minimum TLS protocol to TLSv1.2

Some environments are subject to strict rules about the permitted TLS
protocol verion and available ciphers. Setting TLSv1.2 as the minimum
version ensures we do not use weaker protocols. We've opted against
making this configurable given the age of TLSv1.2 and the increasing
push to deprecate TLSv1.1 and older.

The PreferServerCipherSuites setting is also commonly flagged by SSL
quality scanning tools. Since Go provides a relatively modern set of
default ciphers by default, defaulting this to true is unlikely to
make much practical difference.
Signed-off-by: Steven Danna <steve@chef.io>

Signed-off-by: Stephan Renatus <srenatus@chef.io>

Signed-off-by: Stephan Renatus <srenatus@chef.io>

Dockerfile: update Go to 1.11.5

PR contains connector for openstack keystone.

Features:

    access tokens
    refresh tokens
    groups

Requirements:

    access to openstack keystone instance
    keystone administrative account credentials

Enabling keystone connector specific tests:

    make sure docker is running
    export DEX_TEST_KEYSTONE=1
    make tests

LDAP connector - add emailSuffix config option

Replace "GET", "POST" to http.MethodGet and http.MethodPost

Dockerfile: update to Go 1.11.3

Go 1.11.3 and 1.10.6 were released to mitigate security issues.
These don't appear to impact dex, but update anyway.

Ref: https://groups.google.com/forum/#!topic/golang-announce/Kw31K8G7Fi0

Make expiry of auth requests configurable

This is a band-aid against #1292

I did not change the default from 24h, but I think it should be much lower for safety.