rdp: disable kerberos until settings are available

When trying to connect using a kerberos-enabled client, the connection fails when kerberos authentication is enabled in FreeRDP. This commit disables it by default for now until Kerberos is supported in KRDC.

Previously with FreeRDP2, before commit b5049444, this issue was not present.

Error log without this patch:

KRDC: Scaling changed false
[17:33:25:646] [470366:00072d71] [ERROR][com.winpr.sspi.Kerberos] - [kerberos_InitializeSecurityContextA]: krb5_get_credentials (Server not found in Kerberos database [-1765328377])
[17:33:25:647] [470366:00072d71] [WARN][com.winpr.sspi] - [winpr_InitializeSecurityContextA]: InitializeSecurityContextA status SEC_E_NO_CREDENTIALS [0x8009030E]
[17:33:25:647] [470366:00072d71] [ERROR][com.freerdp.core.auth] - [credssp_auth_authenticate]: InitializeSecurityContext failed with SEC_E_NO_CREDENTIALS [0x8009030E]
[17:33:25:647] [470366:00072d71] [ERROR][com.freerdp.core.rdp] - [rdp_recv_callback_int][0x62991cb89300]: CONNECTION_STATE_NLA - nla_recv_pdu() fail
[17:33:25:647] [470366:00072d71] [ERROR][com.freerdp.core.rdp] - [rdp_recv_callback_int][0x62991cb89300]: CONNECTION_STATE_NLA status STATE_RUN_FAILED [-1]
[17:33:25:647] [470366:00072d71] [ERROR][com.freerdp.core.transport] - [transport_check_fds]: transport_check_fds: transport->ReceiveCallback() - STATE_RUN_FAILED [-1]
[17:33:25:647] [470366:00072d71] [ERROR][com.freerdp.core] - [rdp_client_wait_for_activation]: ERRCONNECT_CONNECT_TRANSPORT_FAILED [0x0002000D]
KRDC: Unable to connect
KRDC: ERRCONNECT_CONNECT_TRANSPORT_FAILED The connection transport layer failed.