Undercloud install fails | DHCP port already in use

Issue

openstack undercloud install fails with the error: failed to bind DHCP server socket: Address already in use:

As seen in install-undercloud.log:

Jan 28 14:29:16 manager systemd: openstack-ironic-inspector-dnsmasq.service: control process exited, code=exited status=2                                                                                                                    
Jan 28 14:29:16 manager systemd: Failed to start PXE boot dnsmasq service for Ironic Inspector.
Jan 28 14:29:16 manager systemd: Unit openstack-ironic-inspector-dnsmasq.service entered failed state.
Jan 28 14:29:16 manager systemd: openstack-ironic-inspector-dnsmasq.service failed.
Jan 28 14:29:19 manager systemd: Starting PXE boot dnsmasq service for Ironic Inspector...
Jan 28 14:29:19 manager dnsmasq: dnsmasq: failed to bind DHCP server socket: Address already in use
Jan 28 14:29:19 manager dnsmasq[124788]: failed to bind DHCP server socket: Address already in use
Jan 28 14:29:19 manager dnsmasq[124788]: FAILED to start up