man 5 passwd states following;
authorS.Çağlar Onur <caglar@verivue.com>
Thu, 17 Mar 2011 17:32:26 +0000 (13:32 -0400)
committerS.Çağlar Onur <caglar@verivue.com>
Thu, 17 Mar 2011 17:32:26 +0000 (13:32 -0400)
commitcfdf17001304aa826b89248cf599f18c8dde51d5
tree0c86414f22b0245c5c1ddfab724803afdd335f9e
parentd6da68e6125ff8acc880989db872f75aed8cdcf1
man 5 passwd states following;

[...]
password  the encrypted user password, an asterisk (*), or the letter 'x'.  (See pwconv(8) for an explanation of 'x'.)
[...]

so handle both situation
config.planetlab/vserver.post