Split run.sh and add git retrival

This patch modularize run.sh, adding two new helper scripts and
make it possible to specify a git repository for ssh keys via a
new env variable `BORG_SSHKEYS_REPO`.

the modularization add two new files :
- `env.sh` : define a few envriroment variables
- `create-client-dirs.sh`  : update and create user directories and
  re-create authorized_keys

We also add a new script `update-ssh-keys.sh` to be called regurlarly
in a cron job to check if the git repository is up-to-date and
eventually adding/removing users.

data/update-ssh-keys.sh

@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -e
+
+source env.sh
+
+if [ -d "${SSH_KEY_DIR}/clients/.git" ] ; then
+  cd "${SSH_KEY_DIR}/clients" || exit 0
+  git fetch
+  if ! git diff --quiet remotes/origin/HEAD; then
+    echo "Pull from git repository"
+    git pull
+    create-client-dirs.sh
+  else
+    echo "$0: Nothing to do"
+  fi
+fi