neutron-ports-plugin: introduce new helper functions

Getting JSON response from oVirt Engine server and Neutron server using
respective helper function.

Signed-off-by: huntxu <mhuntxu@gmail.com>
neutron-ports-plugin: use anonymous functions as callbacks

Signed-off-by: huntxu <mhuntxu@gmail.com>
neutron-ports-plugin: add missing check

Signed-off-by: huntxu <mhuntxu@gmail.com>