Need add empty option in NETWORK_MODE choice for custom system test

[Egor Kotko]

Need add empty option in NETWORK_MODE choice for custom system test

See the screen:
http://snag.gy/AusBo.jpg

[Mateusz Matuszkowiak (mmatuszkowiak)]

So Egor wants here to have option to not set NETWORK_MODE variable at all - not to have it 'empty'.

[Fuel Devops McRobotson (fuel-devops-robot)]

Fix proposed to branch: master
Change author: Dzmitry Moisa <email address hidden>
Review: https://review.fuel-infra.org/14789

[Fuel Devops McRobotson (fuel-devops-robot)]

Reviewed: https://review.fuel-infra.org/14789
Submitter: Aleksandra Fedorova <email address hidden>
Branch: master

Commit: 971a31e8aa9a8d996dc0c061df847eeed7af9aed
Author: Dzmitry Moisa <email address hidden>
Date: Tue Dec 29 10:41:10 2015

Add empty NETWORK_MODE option choice

This commit adds empty option in NETWORK_MODE choice for custom system
test (8.0 and 9.0)

Change-Id: I48a9c035d8bfe17d2af164afde5f661105225d7a
Closes-Bug: #1527218

[ElenaRossokhina (esolomina)]

Verified
8.0.custom_system_test (https://docs.google.com/a/mirantis.com/drawings/d/141NPdVUVT-4un2N5QsUBjSSRE2mw-q3xqpUp7OTB1pA/edit?usp=sharing)
9.0.custom_system_test