Update some links to use https in favor of http

Update all apt.dev.grnet.gr and cdn.synnefo.org links to use https
Fix a bug in snf-image-update-helper

If the helper download command failed, the partially downloaded file
was not cleaned up.
Workaround a cURL bug that triggers SIGPIPE

Ignore SIGPIPE errors in curl to workaround a known bug. Use md5sum to
check the integrity of the downloaded file.

This Fixes #55
Uniformly use PASSWD in variable names

Till now, half of the variable names that referred to passwords contained
PASSWORD in their name and half contained PASSWD
Add a new img_passwd_hash OS parameter

This parameter can be used instead of img_passwd to directly inject a
password hash in the shadow file of the instance. This is applicable
only to Unix images.

This resolves #36
Update the helper kernel

Update the helper kernel to 3.2.60-1+deb7u3+ufs.rw
Add a --with-helper-package-version in configure

This can be used to change the helper package version in the
version_pinning.pref file.
Don't create a partition table in the helper image

There is no need for having partitions in the helper image. It is more
convenient if we format the whole disk as an ext2 filesystem.
Shrink the helper image to the minimum size

Since the helper image is read-only there is no need to waste space.