Description of problem:
In the IPA Server - Configuration tab, User Option section, Default user objectclasses allows deleting and adding objectclasses which then prevents one from adding new users

For example, if ipaobject is deleted, then when adding a new user, it throws error attribute "ipaUniqueID" not allowed. 

So do not allow the listed objectclasses to be deleted...only new ones should be allowed to be added.

When adding new ones, there is no check to see if it is a valid objectclass. Else it throws error when adding a new user

When adding a valid new objectclass, for example mailGroup, it still throws error when adding a new user, indicating - missing attribute "mail" required by object class "mailGroup". This can be worked around in cli by using --setattr=mail="one", and user can be added...but this option is not available in UI

Version-Release number of selected component (if applicable):
ipa-server-2.1.1-4.el6.x86_64

How reproducible:
always

Steps to Reproduce:
1.In the configuration tab - Delete ipaobject from the list of Default user objectclasses
2. Add a new user
3. In the configuration tab - Add mailGroup
4. Add a new user
  
Actual results:
For first user add attempt - throws error - attribute "ipaUniqueID" not allowed
For the second user add attempt - throws error - missing attribute "mail" required by object class "mailGroup"

Expected results:
There should not be a "Delete" button for objectclasses without which new user cannot be added
When adding new objectclass, prompt or allow attribute to be entered when adding new user.

Additional info:
Logged Bug 741951 for cli to indicate an error when deleting required objectClass. But in UI, there should not be a Delete button for these.