file_example_key_management_plugin and example_key_management_plugin don't work with yassl

For some strange reason, when compiling with OpenSSL, more internal encryption functions are exported. These plugins use them directly. So, when compiled with YaSSL these symbols aren't available and plugins don't load.

We need to
1. consistently export or hide (preferrably) these symbols both with OpenSSL and YaSSL
2. export the encryption as a service for key management plugins to use or change plugins not to use it.

When fixed a workaround from plugins' CMakelIsts.txt files should be removed