Specify OpenSSL versions for `conf-libssl`

[liyishuai]

1. async_ssl v0.16.0 requires SSL_set1_groups_list.
2. SSL_set1_groups_list was first introduced in OpenSSL 1.1.1.
3. The current conf-libssl package picks openssl-devel on Fedora:
   

   opam-repository/packages/conf-libssl/conf-libssl.4/opam

   Lines 31 to 33 in 1db3104

   	["openssl-devel"] {os-distribution = "centos"}
   	["openssl-devel"] {os-distribution = "ol"}
   	["openssl-devel"] {os-distribution = "fedora" | os-family = "fedora"}

4. CentOS 7 provides openssl-devel with version 1.0.2k.
5. The proper way to get OpenSSL 1.1 is via the openssl11-devel package on EPEL 7.

The current conf-libssl package doesn't provide any version guarantee for OpenSSL, so it doesn't guarantee successful compilation of dependants.

[mseri]

We could create a new conf-libssl version with the correct package name, however centos7 is no longer supported so we cannot test it.

Otherwise it is currently possible to work around it by installing openssl11-devel by hand and using opam install async_ssl --assume-depexts to skip the depext step