dhcp-all-interfaces: correct ifquery return stmt

Corrects an issue in the config_exists function that could
cause duplicate interfaces to be configured on Ubuntu.

Previously we used 'return ifquery' directly which caused
a silent 'numberic argument required' error to go unnoticed.
This would also return 1 meaning a new interface would get
configured.

The new logic should handle things properly.

Change-Id: I625225e15113d7e184e3bcb5054df1616dec008a
Closes-bug: #1298430
Merge "dhcp-all-interfaces: correct ifquery return stmt"