fixing system test issue where non-existent interface could be selected

[dlyssenko]

1. Dut's API gets list of all interfaces in /pyeapi/api/interfaces.py - it read those from show run all.
2. However, some interfaces present in the 'config all' might not be present on the actual dut.
3. This might create an issue during sytem test if random_interface () picks an interface which is not currently present on the dut.

this PR should fix the issue