Improve provisioning central IP address for ovn tests

Improve provisioning central IP address for ovn tests

Function responsible for providing central ovn address has been
corrected to utilize already configured address instead of
checking it each time. Error reporting within the same function
has been also corrected and improved as well as iproute package
became used for address determining instead of net-tools' ifconfig

Issue-ID: MULTICLOUD-686

Signed-off-by: Konrad Bańka <k.banka@samsung.com>
Change-Id: I156297098b7919471df711f049f0d9439bd81420