gazebo_rtt_plugin: fixed infinite loop if port/property config element has no name attribute and use UpdateTimer class properly

Note: Using attributes within <plugin>...</plugin> elements is currently broken in gazebo.
See https://bitbucket.org/osrf/gazebo/issue/735/attributes-of-child-elements-are-not.
gazebo_rtt_plugin: also accept sub-elements instead of attributes in <port> configuration elements due to a bug in Gazebo (see https://bitbucket.org/osrf/gazebo/issue/735/attributes-of-child-elements-are-not)