gui test: update gui_elements.py
gui test: add test_topology_discavery
gui test: add test_link_discavery
gui test: add test_flow_discavery
gui test: fix test_contents_draggable
gui test: fix typo discavery -> discovery
gui test: refactoring

- gui_elements.py: wait_for_*
- gui_test.py: REST request
test-requires: selenium for GUI test
Merge pull request #40 from yuta-hamada/gui-test-discavery

Gui test discavery