add MultiKernelManager.default_kernel_name configurable

sets the kernel name to use, if unspecified
allow kernel_name to be undefined in requests

fallback to KM.default_kernel_name in that case
allow kernel_name to be undefined in js

Falls back to KM.default_kernel_name,
as configured server-side.
Merge pull request #6568 from minrk/remember-kernel

Make default kernel configurable