change jenkins' umask to 022

Previously, the umask was not correctly propagated to jenkins by the
init script, so this setting was ineffective. Now it changed to 027
breaking at least rebootstrap jobs by turning
/etc/apt/apt.conf.d/80proxy to -rw-r----- in chroot-run.sh.

etc/default/jenkins

@@ -42,7 +42,7 @@ MAXOPENFILES=8192
 # set the umask to control permission bits of files that Jenkins creates.
 #   027 makes files read-only for group and inaccessible for others. comment this out to inherit setting
 #   (as of Ubuntu 12.04, by default umask comes from pam_umask(8) and /etc/login.defs
-UMASK=027
+UMASK=022
 
 # port for HTTP connector (default 8080; disable with -1)
 HTTP_PORT=8080