Use 2 network clients from tempest-lib

The files below have been migrated to tempest-lib

 * agents_client.py    : I97b67e2385452db6ef59bc4b8c63c211ca119bc2
 * extensions_client.py: I76b1d0b1c3ffb2c366c4e6d95105d9c8fde2a22b

Now Tempest-lib provides those as stable interfaces. So Tempest should
start using those from lib and remove its own copy.

Change-Id: Ia2b48d9f2848d006cd9734c7fb5c2ea65c13e043
3 files changed