DHCP agent should check interface is UP before adding route

The DHCP agent should check not only that an interface for
network's DHCP exists but also make sure that is UP before
adding a default route.
For this purpose a method "ensure_device_is_ready" was
added to ip_lib.

Change-Id: I9af06aa0f39634fe7b63c064337cd4191db5c026
Closes-bug: #1302312
Merge "DHCP agent should check interface is UP before adding route"