Description of problem:
IPA installation should check Relative Distinguished Name (RDN) before
installation rather than failing after process start

ipa-server-install --setup-dns --forwarder=10.16.36.29  -r TESTRELM.TEST -p
Secret123 -a Secret123 --subject=NOSUBJECT -U

<snip>

WARNING: conflicting time&date synchronization service 'chronyd' will be
disabled
in favor of ntpd

Warning: skipping DNS resolution of host ipaserver.testrelm.test
The domain name has been determined based on the host name.

Checking DNS forwarders, please wait ...
Using reverse zone(s) 121.65.10.in-addr.arpa.

The IPA Master Server will be configured with:
Hostname:       ipaserver.testrelm.test
IP address(es): 10.65.121.12
Domain name:    testrelm.test
Realm name:     TESTRELM.TEST

BIND DNS server will be configured to serve IPA domain with:
Forwarders:    10.16.36.29
Reverse zone(s):  121.65.10.in-addr.arpa.

Configuring NTP daemon (ntpd)
  [1/4]: stopping ntpd
  [2/4]: writing configuration
  [3/4]: configuring ntpd to start on boot
  [4/4]: starting ntpd
Done configuring NTP daemon (ntpd).
ipa.ipapython.install.cli.install_tool(Server): ERROR    malformed RDN string =
"NOSUBJECT"




Version-Release number of selected component (if applicable):
ipa-server-4.2.0-15.el7.x86_64

How reproducible:
100%

Steps to Reproduce:
1. Start fresh installation of IPA server with invalid RDN as subject argument.

Actual results:
Error message after installation process
ipa.ipapython.install.cli.install_tool(Server): ERROR    malformed RDN string =
"NOSUBJECT"

Expected results:
Error message before installation starts with informative/warning message to
user rather than "malformed RDN string"