Add IP addresses filtering

Add IP addresses filtering

Each node might be described with 3 types of addresses [1]. Some
providers also use node annotations [2] for assigned addresses.

This patch filters out all IP addresses from nodes list. External IPs
take precedence over internal ones. The first address on the extracted
slice will be later used to run the scan on.

This behaviour could be later modified to e.g. loop over all extracted
IP addresses (if scan fails).

[1] https://kubernetes.io/docs/concepts/architecture/nodes/#addresses
[2] https://github.com/rancher/rke/blob/master/k8s/node.go#L18

Issue-ID: SECCOM-261
Change-Id: Ifd094447f778da378dfe1aee765f552b6ebd669f
Signed-off-by: Pawel Wieczorek <p.wieczorek2@samsung.com>