Handle the case where the DS group exists but the user does not

If the group exists but the user doesn't then useradd blows up
trying to create the user and group. So test to see if the group
exists and if it does pass along the -g argument to useradd.

Resolves #502960

ipaserver/install/dsinstance.py

@@ -21,6 +21,7 @@
 import shutil
 import logging
 import pwd
+import grp
 import glob
 import sys
 import os
@@ -217,6 +218,14 @@ class DsInstance(service.Service):
             user_exists = False
             logging.debug("adding ds user %s" % self.ds_user)
             args = ["/usr/sbin/useradd", "-c", "DS System User", "-d", "/var/lib/dirsrv", "-M", "-r", "-s", "/sbin/nologin", self.ds_user]
+            try:
+                # if the group already exists we need to request to add it,
+                # otherwise useradd will create it for us
+                grp.getgrnam(self.ds_user)
+                args.append("-g")
+                args.append(self.ds_user)
+            except KeyError:
+                pass
             try:
                 ipautil.run(args)
                 logging.debug("done adding user")