configverify: T6642: verify_interface_exists requires config_dict arg

The function verify_interface_exists requires a reference to the ambient
config_dict rather than creating an instance. As access is required to
the 'interfaces' path, provide as attribute of class ConfigDict, so as
not to confuse path searches of script-specific config_dict instances.

(cherry picked from commit 5f23b7275564cfaa7c178d320868b5f5e86ae606)