Move jenkins cache to /opt/cache

For whatever reason, the source-repositories caching element is
running in the very early extra-data phase of disk-image-builder.
cache-devstack is instructing source-repositories to put the cached
urls into /home/jenkins/cache/files (55-cache-devstack-repos makes up
the list of images to download from devstack; the other two are just
pre-configured URLS).

This causes a problem when trying to add the jenkins user later in
jenkins-slave/install.d/20-jenkins-slave.  Since source repositories
has already created /home/jenkins, useradd won't do it again and does
not copy in any of the skeleton files.

This moves the extra-data phase caching into /opt/cache, which is
probably a more logical place for it anyway.  To maintain the status
quo, after we create the jenkins user, symlink ~jenkins/cache/files to
this new location.

This was probably going wrong with puppet too.  I'm guessing it just
ignored the existing homedir when creating the user.  I have tested
this by building trusty and stopping the build before and after the
useradd calls in 20-jenkins-slave.  "/home" was empty, the useradd
creates the user, group & directory, the symlink is made correctly and
/opt/cache/files appears populated as usual.

Change-Id: Ibca6867f29b257a5110cb5522a5cca3a97fa9377

nodepool/elements/cache-devstack/extra-data.d/55-cache-devstack-repos

@@ -160,7 +160,7 @@ def main():
                 image_filenames.append(fname)
                 args = dict(
                     name=fname,
-                    location=os.path.join('/home/jenkins/cache/files', fname),
+                    location=os.path.join('/opt/cache/files', fname),
                     url=url)
 
                 images_list.write(line_template % args)

nodepool/elements/cache-devstack/source-repository-pip

@@ -1 +1 @@
-get-pip-py file /home/jenkins/cache/files/get-pip.py https://bootstrap.pypa.io/get-pip.py
+get-pip-py file /opt/cache/files/get-pip.py https://bootstrap.pypa.io/get-pip.py

nodepool/elements/cache-devstack/source-repository-stackviz

@@ -1 +1 @@
-stackviz file /home/jenkins/cache/files/stackviz-latest.tar.gz http://tarballs.openstack.org/package-stackviz-element/stackviz-latest.tar.gz
+stackviz file /opt/cache/files/stackviz-latest.tar.gz http://tarballs.openstack.org/package-stackviz-element/stackviz-latest.tar.gz

nodepool/elements/jenkins-slave/install.d/20-jenkins-slave

@@ -6,9 +6,18 @@ fi
 set -eu
 set -o pipefail
 
+# Add jenkins user and group.  Note we don't want to rely on
+# "useradd"'s group adding behaviour, because it might differ across
+# distros.
 groupadd jenkins
 useradd -g jenkins -m jenkins -s /bin/bash
 
+# a lot of caching happens in extra-data.d (for "historical" reasons).
+# We've put the cache stuff into /opt/cache/files, but again, for
+# "historical" reasons, ensure this is available in /home/jenkins
+mkdir -p /home/jenkins/cache
+ln -sf /opt/cache/files /home/jenkins/cache/files
+
 # this was copied from outside the chroot by extras.d
 _pub_key=/tmp/in_target.d/jenkins-user-ssh-public-key
 if [ ! -f $_pub_key ]; then