[FIXED JENKINS-24514]

Ubuntu (at least as of 12.04) has the default umask 022, which made some
users nervous. Quoting its /etc/login.defs below, which explains its
historical origin:

  UMASK is the default umask value for pam_umask and is used by
  useradd and newusers to set the mode of the new home directories.
  022 is the "historical" value in Debian for UMASK
  027, or even 077, could be considered better for privacy
  There is no One True Answer here : each sysadmin must make up his/her
  mind.

It does seem to me that a bit more restrictive default is sensible,
so this change introduces /etc/default/jenkins parameter that sets the
default umask to 027 to prevent "others" from seeing files.

Not that keys and other sensitive files are protected anyway, so it is
not the case that the privacy of Jenkins data files have been vulnerable
prior to this change.