Allow source-repositories to be disabled completely

In our official image builds we are only allowed to use resources
that are "blessed" by the build system.  This means that external
things like git repos and tar files are not allowed.  Currently,
even in offline mode source-repositories expects those things to
be available in the cache, so we need a way to disable it entirely.

This change adds an environment variable NO_SOURCE_REPOSITORIES
that does so.  It can be set in an environment.d script so elements
that might rely on a source repository will know it's not available.
The 02-lsb script in redhat-common is one such example and is
updated to handle this case.

Change-Id: I0de63bee6ad79733d6711478c707a9b41593e85f

elements/redhat-common/pre-install.d/02-lsb

@@ -3,4 +3,8 @@
 set -eu
 set -o pipefail
 
+if [ -z "${NO_SOURCE_REPOSITORIES:-}" ]; then
     install -m 0755 -o root -g root /opt/stack/lsb-release/lsb_release /usr/local/bin
+else
+    install-packages redhat-lsb-core
+fi

elements/source-repositories/README.md

@@ -103,3 +103,7 @@ The base url for all git repositories can be set by use of:
 So setting DIB\_GITREPOBASE=https://github.com/ when the repo location is set
 to http://git.openstack.org/openstack/nova.git will result in use of the
 https://github.com/openstack/nova.git repository.
+
+When doing image builds in environments where external resources are not allowed,
+it is possible to disable fetching of all source repositories by including an
+element in the image that sets NO_SOURCE_REPOSITORIES=1 in an environment.d script.

elements/source-repositories/extra-data.d/98-source-repositories

@@ -3,6 +3,11 @@
 set -eu
 set -o pipefail
 
+# Don't provide any source repositories in environments where they are not allowed
+if [ -n "${NO_SOURCE_REPOSITORIES:-}" ]; then
+    exit 0
+fi
+
 # If the old cache exists, move it to the new name
 function make_new_cache(){
     local OLD_CACHE_BASE=$1