kra-install: better warning message

User would like to see CA installation command in KRA installation
warning message.

This makes warning message similar to other installer messages where it
does suggests a command to run.

https://pagure.io/freeipa/issue/6952

Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
Reviewed-By: Felipe Barreto <fbarreto@redhat.com>

ipaserver/install/ipa_kra_install.py

@@ -150,7 +150,8 @@ class KRAInstaller(KRAInstall):
 
         if not cainstance.is_ca_installed_locally():
             raise RuntimeError("Dogtag CA is not installed. "
-                               "Please install the CA first")
+                               "Please install a CA first with the "
+                               "`ipa-ca-install` command.")
 
         # check if KRA is not already installed
         _kra = krainstance.KRAInstance(api)