dtls_echo_server.c: force use of AES ciphers
dtls_echo_server.c: force use of AES ciphers

Summary: - Revert "Merge pull request #181 from AVSystem/coap-ctx-dtls-test-disable"

Test Plan: `make avs_coap_check` on Ubuntu 16.04 & Zygfryd

Reviewers: kfyatek!, mkrawiec, kpanek, molczyk

Differential Revision: https://phabricator.avsystem.com/D7326

AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons,AVSystem/avs_commons