It reads CA certificates from:
* The file in environment variable SSL_CERT_FILE
* The first available file of
/etc/ssl/certs/ca-certificates.crt (Debian/Ubuntu/Gentoo etc.)
/etc/pki/tls/certs/ca-bundle.crt (Fedora/RHEL 6)
/etc/ssl/ca-bundle.pem (OpenSUSE)
/etc/pki/tls/cacert.pem (OpenELEC)
/etc/pki/ca-trust/extracted/pem/tls-ca-bundle.pem (CentOS/RHEL 7)
/etc/ssl/cert.pem (Alpine Linux)
* Files in the directory of environment variable SSL_CERT_DIR
* Files in the first available directory of
/etc/ssl/certs (SLES10/SLES11, https://golang.org/issue/12139)
/etc/pki/tls/certs (Fedora/RHEL)
/system/etc/security/cacerts (Android)
