KNOX-440: HttpFS impersonation issue

KNOX puts 'doAs' parameter to impersonate certain user. However, HttpFS does not accept it; it accepts 'doas' instead. According to WebHDFS REST API specification (http://hadoop.apache.org/docs/stable/hadoop-project-dist/hadoop-hdfs/WebHDFS.html#Proxy_Users), 'doas' is the correct one.