Changes to use 2048 bit public key for self-signed certificate

After 1/1/2014, 1024 bit keys are no longer considered secure enough
and may not be supported by all browsers. So changes to use 2048 bit
public key.

See Appendix A in this document:
https://www.cabforum.org/Baseline_Requirements_V1.pdf

Signed-off-by: Mark Wu <wudxw@linux.vnet.ibm.com>