Allow disto-specific mirror settings

Most of our distros support DIB_DISTRIBUTION_MIRROR for setting the
mirror to be used when building. This has a major downside where if
someone would like to create a static configuration for building various
distro images it cannot set DIB_DISTRIBUTION_MIRROR (think of the case
where we want to run our tests).

By adding support for DIB_$DISTRO_DISTRIBUTION_MIRROR this problem is
resolved.

Change-Id: I92964b17ec3e47cf97e3a3091f054b2a205ac768

elements/centos/environment.d/10-centos6-distro-name.bash

@@ -3,3 +3,7 @@ export DIB_RELEASE=${DIB_RELEASE:-GenericCloud}
 
 # Useful for elements that work with fedora (dnf) & centos
 export YUM=${YUM:-yum}
+
+if [ -n "${DIB_CENTOS_DISTRIBUTION_MIRROR:-}" ]; then
+    export DIB_DISTRIBUTION_MIRROR=$DIB_CENTOS_DISTRIBUTION_MIRROR
+fi

elements/centos/pre-install.d/02-yum-repos

@@ -6,10 +6,12 @@ fi
 set -eu
 set -o pipefail
 
+DIB_DISTRIBUTION_MIRROR=${DIB_DISTRIBUTION_MIRROR:-http://mirror.centos.org/centos}
+
 cat << EOF > /etc/yum.repos.d/centos6-latest.repo
 [rhel6]
 name=centos6
-baseurl=http://mirror.centos.org/centos/6/os/x86_64/
+baseurl=$DIB_DISTRIBUTION_MIRROR/6/os/x86_64/
 enabled=1
 metadata_expire=7d
 gpgcheck=0

elements/centos7/environment.d/10-centos7-distro-name.bash

@@ -3,3 +3,7 @@ export DIB_RELEASE=GenericCloud
 
 # Useful for elements that work with fedora (dnf) & centos
 export YUM=${YUM:-yum}
+
+if [ -n "${DIB_CENTOS_DISTRIBUTION_MIRROR:-}" ]; then
+    export DIB_DISTRIBUTION_MIRROR=$DIB_CENTOS_DISTRIBUTION_MIRROR
+fi

elements/debian-minimal/environment.d/10-debian-minimal.bash

@@ -1,6 +1,11 @@
 export DISTRO_NAME=debian
 export DIB_RELEASE=${DIB_RELEASE:-stable}
+
+if [ -n "${DIB_DEBIAN_DISTRIBUTION_MIRROR:-}" ]; then
+    DIB_DISTRIBUTION_MIRROR=$DIB_DEBIAN_DISTRIBUTION_MIRROR
+fi
 export DIB_DISTRIBUTION_MIRROR=${DIB_DISTRIBUTION_MIRROR:-http://ftp.us.debian.org/debian}
+
 export DIB_DEBIAN_COMPONENTS=${DIB_DEBIAN_COMPONENTS:-main}
 export DIB_DEBIAN_COMPONENTS_WS=${DIB_DEBIAN_COMPONENTS//,/ }
 

elements/fedora/environment.d/10-fedora-distro-name.bash

@@ -1,2 +1,5 @@
 export DISTRO_NAME=fedora
 export DIB_RELEASE=${DIB_RELEASE:-24}
+if [ -n "${DIB_FEDORA_DISTRIBUTION_MIRROR:-}" ]; then
+    export DIB_DISTRIBUTION_MIRROR=$DIB_FEDORA_DISTRIBUTION_MIRROR
+fi

elements/ubuntu-minimal/environment.d/10-ubuntu-distro-name.bash

@@ -1,4 +1,8 @@
 export DISTRO_NAME=ubuntu
 export DIB_RELEASE=${DIB_RELEASE:-xenial}
 export DIB_DEBIAN_COMPONENTS=${DIB_DEBIAN_COMPONENTS:-main,restricted,universe}
+
+if [ -n "${DIB_UBUNTU_DISTRIBUTION_MIRROR:-}" ]; then
+    DIB_DISTRIBUTION_MIRROR=$DIB_UBUNTU_DISTRIBUTION_MIRROR
+fi
 export DIB_DISTRIBUTION_MIRROR=${DIB_DISTRIBUTION_MIRROR:-http://archive.ubuntu.com/ubuntu}