Description of problem:
ipa server-del fails when user does not provide server name in interactive mode

[root@vm-idm-027 ~]# ipa server-del
Server name:
ipa: ERROR: KeyError: 'cn'
Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/ipalib/cli.py", line 1351, in run
    sys.exit(api.Backend.cli.run(argv))
  File "/usr/lib/python2.7/site-packages/ipalib/cli.py", line 1109, in run
    kw = self.process_keyword_arguments(cmd, kw)
  File "/usr/lib/python2.7/site-packages/ipalib/cli.py", line 1096, in
process_keyword_arguments
    callback(cmd, kw)
  File "/usr/lib/python2.7/site-packages/ipaclient/plugins/server.py", line 17,
in interactive_prompt_callback
    "please wait...") % {'servers': ', '.join(kw['cn'])})
KeyError: 'cn'
ipa: ERROR: an internal error has occurred



Version-Release number of selected component (if applicable):
ipa-server-4.4.0-8.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. ipa server-del # in interactive mode press enter after Server Name:

Actual results:
Stack trace produced by command

Expected results:
Command should notify user about blank server name