Fixing neutron::server connection parameter

Previously, neutron::server's (deprecated) sql_connection parameter
had a default value, preventing the new/preferred connection
parameter from ever being used.

This change removes that default, permitting the connection
parameter to work properly, and extends unit tests to confirm.

bug introduced at commit 82552e79316a4ffc1c879dd3aaa1f4735a4a35a0,
change I300fc12092bca1abdd105e0c0c272f7fd49258ac

Change-Id: I116b482c620b5c44b88c1b801438a100343f6f9f
Merge "Fixing neutron::server connection parameter"