ipalib: remove hard dependency on ipapython

Hard-code the path to /bin/false in SubprocessError doc string.

Remove ipaplatform dependency from ipalib's setup.py and add it as optional
installer dependency to ipalib's and ipaclient's setup.py.

https://fedorahosted.org/freeipa/ticket/6474

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

ipaclient/setup.py

@@ -55,6 +55,7 @@ if __name__ == '__main__':
             "wheel",
         ],
         extra_requires={
+            "ipaclient.install": ["ipaplatform"],
             "otptoken_yubikey": ["yubico", "usb"]
         }
     )