Fix checking of public IPs count

Fixed CheckBeforeDeploymentTask._check_network() method to properly
compare quantity of public IPs and nodes that require them.
Tests added.

Closes-Bug: #1376426

Change-Id: I9b3278286a9d67aff784e5c241db1dc299a2f94e
(cherry picked from commit df80eb24b159d565170cd5faff94745713ccb8ea)