Added test to check list/show extensions-neutron

Added a test script 'test_extensions.py' to api/networks to verify
if list/show of extensions is successful in neutron. Also added
supporting methods 'list_extensions' and 'show_extension_details' in
network client for XML and JSON interfaces.

Implements blueprint: quantum-test-l2-abstraction

Change-Id: Ie939a3e1302060489b579baf2cd3f7023efbeb7d
Merge "Added test to check list/show extensions-neutron"