The variable 'ironic_dnsmasq_interface' is used to configure the interface
used by the ironic inspector dnsmasq service for DHCP on the inspection
network. It is being used correctly in inspector.conf, but not in the
dnsmasq configuration file, which uses api_interface. This change modifies
the dnsmasq configuration file to also use ironic_dnsmasq_interface.

Change-Id: I7670544f4bc41c93ac1d081486502f9ffb8f2286
Closes-Bug: #1785574