[#2718] Add private key decryption to JdkSslServerContext

Backported: d9cccccbb3344997e016e6a3603126ce65705c4d

Motivation:

Currently it is not possible to load an encrypted private key when
creating a JDK based SSL server context.

Modifications:

- Added static method to JdkSslServerContext which handles key spec
  generation for (encrypted) private keys and make use of it.
- Added tests for creating a SSL server context based on a (encrypted)
  private key.

Result:

It is now possible to create a JDK based SSL server context with an
encrypted (password protected) private key.