CI : travis-ci : prepare for using stock OpenSSL

CI : travis-ci : prepare for using stock OpenSSL

initially SSL_LIB and SSL_INC were set globally and we assumed
that any OpenSSL variant is supposed to be built using "script/build-ssl.sh".

starting with ARM64 build we use stock openssl, also it makes sense
to use stock openssl for 1.1.1 builds for velocity sake.

Let us make stock openssl lib first class citizen.

SSL_LIB and SSL_INC are only set when custom openssl variant
is built.