tests/unit: refactor reading neutron.conf.test

neutron.conf.test includes rpc_backend whose value is stashed.
Thus it is required to reset when tearing down, otherwise the stale status
will be used by succeeding tests causing random error.

This patch refactors reading neutron.conf.test and resets the status of
rpc_backend properly.

Closes-Bug: #1281481
Closes-Bug: #1284549
Change-Id: I0fa5945b6adbb9945d353028ec88d00ccbf4e31a
Merge "tests/unit: refactor reading neutron.conf.test"