rework text on password/email entry step

rework text on password/email entry step

Reworked this text with the help of Daniela.

Change the following:
* Elaborated for what the email gets used.
* Rework the (paraphrased) "we are based on Debian, therefore your
  password needs to be 5 characters or longer" sentence. it didn't
  made sense, in our opinion.
* tell the users that he sets the root password, while most would
  expect this, users new to Linux, or coming from the windows world
  may not.
* move the "Press the Next button [...]" to the right side.

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Daniela Häsler <daniela@proxmox.com>