Allow Windows users to specify OpenSSL options in the ownCloud config

PHP's OpenSSL support compiles in the location of the config file.  It is typically difficult to specify an alternative location in Windows IIS via environment variables.  This PR allows the location of the openssl.cnf (and potentially other options) to be specified in the ownCloud config file.  If no config option is provided, the defaults are used, which work as before (and succeeds on Linux).