ifquery doesn't exist on Fedora

Updates dhcp-all-interfaces so that we avoid using
ifquery on Fedora to determine if an interface exists.

Adds a new config_exists function which simply looks for
the ifcfg config file on distro's that use netscripts.

Change-Id: Ie55524b83820fe4fab28cc9d3f7e08ca2e42c182
Merge "ifquery doesn't exist on Fedora"