utils_net: Add new function to the class Bridge() for STP status

1) improve the return of function Bridge().get_structure()
2) add function to get the status of STP for specified bridge
3) make corresponding modification for list_br() and list_iface()
test_getstructure: check Bridge.get_structure()'s return as new format
Merge pull request #1865 from PandaWei/develop

utils_net: add function to get the status of STP for specified bridge