Don't assume KRB5CCNAME is in the environment in replica install

The replica install was unilaterally removing KRB5CCNAME from
os.environ in some cases. Instead check first to see if it is
present and only remove in that case.

Fixes: https://pagure.io/freeipa/issue/9446

Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>
Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>

ipaserver/install/server/replicainstall.py

@@ -1002,7 +1002,7 @@ def promote_check(installer):
                 raise errors.ACIError(info="Not authorized")
 
             if installer._ccache is None:
-                del os.environ['KRB5CCNAME']
+                os.environ.pop('KRB5CCNAME', None)
             else:
                 os.environ['KRB5CCNAME'] = installer._ccache
 
@@ -1211,7 +1211,7 @@ def promote_check(installer):
         if add_to_ipaservers:
             # use user's credentials when the server host is not ipaservers
             if installer._ccache is None:
-                del os.environ['KRB5CCNAME']
+                os.environ.pop('KRB5CCNAME', None)
             else:
                 os.environ['KRB5CCNAME'] = installer._ccache