Implement ip_lib.device_exists_with_ip_mac

ip_lib.device_exists simply checks that a device is in an
(optional) namespace. This patch adds
ip_lib.device_exists_with_ip_mac, which also verifies that
the device is configured with the given IP and MAC addresses.
The new function is used in functional tests in a patch
further down the dependency chain.

Added functional tests for ip_lib.device_exists,
ip_lib.device_exists_with_ip_mac.

Implements: blueprint l3-high-availability
Change-Id: I1495bcf1f1a88e2eb78df79cccb64121ccf435fb
Merge "Implement ip_lib.device_exists_with_ip_mac"