samba-tool user getpassword: Prepare to support a ;previous=1 option, change behaviou...

samba-tool user getpassword: Prepare to support a ;previous=1 option, change behaviour for ;rounds=

This will return the previous password, but the pattern is to include
the option in the returned attribute name, so we need to use
vatter["raw_attr"], not 'a'.

This changes the behaviour for the ;rounds= option used when we hold
the plaintext password (possibly under GPG encryption).

This is now consistant with other parameters in the LDAP attribute,
and is now included in the returned attribute name.

Signed-off-by: Andrew Bartlett <abartlet@samba.org>
Reviewed-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>