Add support for client certificates for registries

This lets you specify custom client TLS certificates and CA root for a
specific registry hostname. Docker will then verify the registry
against the CA and present the client cert when talking to that
registry.  This allows the registry to verify that the client has a
proper key, indicating that the client is allowed to access the
images.

A custom cert is configured by creating a directory in
/etc/docker/certs.d with the same name as the registry hostname. Inside
this directory all *.crt files are added as CA Roots (if none exists,
the system default is used) and pair of files <filename>.key and
<filename>.cert indicate a custom certificate to present to the registry.

If there are multiple certificates each one will be tried in
alphabetical order, proceeding to the next if we get a 403 of 5xx
response.

So, an example setup would be:
/etc/docker/certs.d/
└── localhost
    ├── client.cert
    ├── client.key
    └── localhost.crt

A simple way to test this setup is to use an apache server to host a
registry. Just copy a registry tree into the apache root, here is an
example one containing the busybox image:
  http://people.gnome.org/~alexl/v1.tar.gz

Then add this conf file as /etc/httpd/conf.d/registry.conf:

 # This must be in the root context, otherwise it causes a re-negotiation
 # which is not supported by the tls implementation in go
 SSLVerifyClient optional_no_ca

 <Location /v1>
 Action cert-protected /cgi-bin/cert.cgi
 SetHandler cert-protected

 Header set x-docker-registry-version "0.6.2"
 SetEnvIf Host (.*) custom_host=$1
 Header set X-Docker-Endpoints "%{custom_host}e"
 </Location>

And this as /var/www/cgi-bin/cert.cgi

 #!/bin/bash
 if [ "$HTTPS" != "on" ]; then
     echo "Status: 403 Not using SSL"
     echo "x-docker-registry-version: 0.6.2"
     echo
     exit 0
 fi
 if [ "$SSL_CLIENT_VERIFY" == "NONE" ]; then
     echo "Status: 403 Client certificate invalid"
     echo "x-docker-registry-version: 0.6.2"
     echo
     exit 0
 fi
 echo "Content-length: $(stat --printf='%s' $PATH_TRANSLATED)"
 echo "x-docker-registry-version: 0.6.2"
 echo "X-Docker-Endpoints: $SERVER_NAME"
 echo "X-Docker-Size: 0"
 echo

 cat $PATH_TRANSLATED

This will return 403 for all accessed to /v1 unless *any* client cert
is presented. Obviously a real implementation would verify more details
about the certificate.

Example client certs can be generated with:

openssl genrsa -out client.key 1024
openssl req -new -x509 -text -key client.key -out client.cert

Docker-DCO-1.1-Signed-off-by: Alexander Larsson <alexl@redhat.com> (github: alexlarsson)
docker daemon: initialize the daemon pidfile early

fixes https://github.com/dotcloud/docker/issues/6973

Docker-DCO-1.1-Signed-off-by: Vincent Batts <vbatts@redhat.com> (github: vbatts)
Change HTTP code: 404 to a more user friendly Image Not found msg

Docker-DCO-1.1-Signed-off-by: William Henry <whenry@redhat.com> (github: ipbabble)
Add Content-Type header in PushImageLayerRegistry

Docker-DCO-1.1-Signed-off-by: Gabor Nagy <mail@aigeruth.hu> (github: Aigeruth)
Added :neckbeard: RMS the founder of :water_buffalo: GNU and :free: as in :free:dom software

Docker-DCO-1.1-Signed-off-by: Aleksandrs Fadins <aleks@s-ko.net> (github: muchweb)
Removed :neckbeard: RMS test case

Docker-DCO-1.1-Signed-off-by: Aleksandrs Fadins <aleks@s-ko.net> (github: muchweb)
archive/tar: update to fix writing of PAX headers

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
contrib/init/systemd: add philips as MAINTAINER

As requested after #7021 add me as a maintainer alongside the sword
toting @lsm5.

Docker-DCO-1.1-Signed-off-by: Brandon Philips <brandon.philips@coreos.com> (github: philips)
integcli: add JSON utils for testing

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
integcli: fix map randomization failures

Docker-DCO-1.1-Signed-off-by: Cristian Staretu <cristian.staretu@gmail.com> (github: unclejack)
Updated Red Hat Enterprise Linux installation instructions for version 7

Docker-DCO-1.1-Signed-off-by: Brian Exelbierd <bex@pobox.com> (github: bcexelbi)
Update docs on --sig-proxy to indicate that SIGKILL and SIGSTOP cannot be proxied

Docker-DCO-1.1-Signed-off-by: Matt Heon <mheon@redhat.com> (github: mheon)
Improve data volume container example.
Docker-DCO-1.1-Signed-off-by: Jason Livesay <ithkuil@gmail.com> (github: runvnc)
Update runmetrics.md
Update libcontainer to cf45d141db69ce11dcccac178e5

Docker-DCO-1.1-Signed-off-by: Michael Crosby <michael@docker.com> (github: crosbymichael)
Merge pull request #7107 from bkochendorfer/patch-1

Update runmetrics.md
Merge pull request #3070 from alexlarsson/certificates
Merge pull request #7043 from ipbabble/404NotFound
Merge pull request #7109 from crosbymichael/update-libcontainer-july4

Update libcontainer to cf45d141db69ce11dcccac178e5
Remove blank "use" documentation

Introduced in dotcloud/docker#3070

Docker-DCO-1.1-Signed-off-by: Ben Firshman <ben@firshman.co.uk> (github: bfirsh)