Added test case to check floating IP API operations

Added a test case by the name
'test_create_list_show_update_delete_floating_ip' to check
floating IP API operations. Also added supporting methods
in network_client.py

Implements blueprint: quantum-test-l2-abstraction

Change-Id: I13ca20a249e9eba0a52e6eb3c6e82baaa38fc5a0
Merge "Added test case to check floating IP API operations"