Description of problem:
When modifying an existing permission to just modify its permissions, it
prompts for all parameters to be entered again

1> Add a permission to have read and write permissions:
# ipa permission-add ManageUser --permissions="read,write" --type=user
--attr=carlicense,description

-----------------------------
Added permission "ManageUser"
-----------------------------
  Permission name: ManageUser
  Permissions: read, write
  Attributes: carlicense, description
  Type: user


2> Modify the permission to have just read permission:
# ipa permission-mod ManageUser --permissions=read
[Attributes]: carlicense
[Type]: user
[Member of group]:
[Filter]:
[Subtree]:
[Target group]:
--------------------------------
Modified permission "ManageUser"
--------------------------------
  Permission name: ManageUser
  Permissions: read
  Attributes: carlicense
  Type: user

Had to enter all parameters again.

Version-Release number of selected component (if applicable):
freeipa-server-2.1.4-4.fc16.x86_64

How reproducible:
always

Steps to Reproduce:
1. Add a permission as above
2. Modify this permission - as above


Actual results:
have to re-enter all parameters, even those that are not changing.

Expected results:
Modify permission based on what is provided in the command, and not prompt for
all attr.

Additional info: