ubuntu-1204.preseed: avoid bashisms

Provisioning fails with kernel 3.13 because of preseed/late_command errors.
These errors are caused by bashisms (echo -e and similar). The default shell
in Ubuntu is dash, also bash is not available in the installer environment
(which uses busybox shell).

Change-Id: Ia47ffe0584424e67f173ca9467334679071053c7
Related-bug: #1390044
Related-bug: #1383641
Merge "ubuntu-1204.preseed: avoid bashisms"