Removing keystore way to provide Ceritficate to leshan-server-demo.

[sbernard31]

Currently there is 2 way to provide certificate to leshan-server-demo :

• KeyStore (which is currently deprecated)
• der encoded (and not so official pem file)

For a demo I think we should limit ourself to only 1 way and so keep the der/pem encoded way :

1. I feel that most people are more comfortable with openssl (maybe I'm wrong)
2. I would prefer to not multiply the way to provide certificate/keys materials.
   2.1 because this means more documentation
   2.2 more code to maintain
   2.3 more use-case to test
3. For a demo, I think we don't care to have unprotected private key. (which is one of the benefits to use KeyStore)

Just to be clear, this is just about demo, at Leshan library level, we will still be able to use KeyStore as we use java abstraction and so we don't care from where come Certificate/PublicKeyPrivateKey. So this is really just a Leshan demo question.

[sbernard31]

This is remove in master with #1158