Description of problem:

WebUI and command line support for multiple managers does not exist.
Need to add support for multiple managers at command line and use the correct
widget in UI

Version-Release number of selected component (if applicable):
FreeIPA, version: 4.1.4

How reproducible:
Every Time

Steps to Reproduce:
  Command line:
  1. Execute 'ipa user-mod foo --manager=aaa --manager=bbb'

  WebUI:
  1. Attempt to assign more than one manager to a user

Actual results:
  Command line:
  ipa: ERROR: invalid 'manager': Only one value is allowed

  WebUI:
  No additional fields present to set/select managers

Expected results:
  Command line:
  Command completes successfully showing modified user with multiple managers
in list.

  WebUI:
  Prompt appears to allow selecting multiple managers from list similar to the
group membership for users prompt.

Additional info:

  Per freenode IRC discussion with users rcrit and ab editing the file at
/usr/lib/python2.7/site-packages/ipalib/plugins/user.py in my environment and
changing line 593 from manager? to manager* fixes the command line issue.

  The WebUI should be using the association widget for managers that is the
same as what is used with group membership selection for users.