Avoid setting empty root password (#964299)

Avoid setting empty root password (#964299)

When using kickstart with no rootpw command, imgcreate ended up calling
"passwd -d root", leaving the root account password-less.  That may lead to
local or remote privilege escalation.

This change does the following:

1) There's no password manipulation done when password is empty string and
   rootpw was not called with --iscrypted
2) Password is locked when "rootpw --lock" is used

Notes:
Users can still shoot themselves in a foot by using: rootpw --iscrypted ""

Resolves: rhbz#964299 (CVE-2013-2069)

Signed-off-by: Brian C. Lane <bcl@redhat.com>