get and check ring size with ethtool

Check ring size with ethtool

Scenarios: X3-ET014, X3-ET015.

1. Run ethtool --show-ring iut_if

2. Check if it is terminated successfully

3. Check if output can be parsed and doesn't print anything to strderr

4. Log obtained ring size

Trempolskii Demid demidtr@oktetlabs.ru

Parameters:

env	Testing environment: env.iut_only