BigSwitch: Fix error for server config check

Raises config error when a port is incorrectly
configured for a server instead of getting a
ValueError when python tries to convert it to
an integer.

Closes-Bug: #1289132
Change-Id: Ifec0bec5640a4a57859f2673c46c610f1d21c36c
Merge "BigSwitch: Fix error for server config check"