user configurable ciphers for communication between Zabbix components

When my 3.2.1 zabbix server starts, it immediately crashes with: cannot set list of PSK ciphersuites: file ssl_lib.c line 1314: error:1410D0B9:SSL routines:SSL_CTX_set_cipher_list:no cipher match

It appears when support for OpenSSL 1.1.0 was added, the minimum ciphersuite selection was reduced from "RSA+aRSA+AES128" to "PSK-AES128-CBC-SHA". My CentOS 6 openssl doesn't support PSK-AES128-CBC-SHA, but it does have 3 supported ciphers that match RSA+aRSA+AES128.

Ideally there would be a TLSCiphers configuration setting that allows specifying them manually with a sane default, but in the near term can we make the ZBX_CIPHERS_PSK define be set to "RSA+aRSA+AES128", which would support the previously cipher suites properly.

(specification)