xen-api: Change PAM configuration from password-auth to passwd
xen-api: Change PAM configuration from password-auth to passwd

password-auth isn't available on Ubuntu, but passwd is and it
seems to behave equivalently to password-auth on CentOS.

In particular, local non-root users can still call xe commands using:

  xe -u root -pw <password> <command>

Signed-off-by: Euan Harris <c3b6e83069c8e9e3af49a38fec6026be89559638@citrix.com>

simonjbeaumont/planex,simonjbeaumont/planex,djs55/planex,simonjbeaumont/planex,djs55/planex,djs55/planex