backup: add backup for /etc/

Adds backup of `/etc/` to `etc/<server>/`. The existing backups of
`/var/lib/jenkins/` are left in `<server>/` to avoid creating new
inodes for unchanged files/directories.

backup/README.md

@@ -8,11 +8,12 @@ currently being backed up.
 
  - ci.nodejs.org:
    - full backup of /var/lib/jenkins (rotated, excluding workdirs)
+   - full backup of /etc (rotated)
    - iptables rules (rotated)
  - ci-release.nodejs.org:
    - full backup of /var/lib/jenkins (rotated, excluding workdirs)
+   - full backup of /etc (rotated)
    - iptables rules (rotated)
- - iojs-softlayer-benchmark: mysql dump (rotated)
  - nodejs-www:
    - /home/dist/iojs: all artifacts for iojs.org (static, no deletion)
    - /home/dist/nodejs: all artifacts for nodejs.org (static, no deletion)

backup/rsnapshot.conf

@@ -21,5 +21,7 @@ ssh_args	-p 22 -i /root/.ssh/nodejs_build_backup
 cmd_preexec	/root/backup_scripts/dist.sh
 backup	root@ci.nodejs.org:/var/lib/jenkins/	ci.nodejs.org/	+rsync_long_args=--exclude "workspace*"
 backup	root@ci-release.nodejs.org:/var/lib/jenkins/	ci-release.nodejs.org/	+rsync_long_args=--exclude "workspace*"
+backup	root@ci.nodejs.org:/etc/	etc/ci.nodejs.org/
+backup	root@ci-release.nodejs.org:/etc/	etc/ci-release.nodejs.org/
 backup	root@direct.nodejs.org:/var/log/nginx/	www-logs/	+rsync_long_args=--exclude "*.log" --exclude "old/*"
 backup_script	/root/backup_scripts/iptables.sh	iptables/