22523c0e42 added role variables that wrap certain mxisd configuration
keys. This has the obvious drawback that not all keys are accessible but
only the wrapped.

This change replaces the previously added wrapper role variables by a
single dictionary variable `matrix_mxisd_extra_config` in which arbirary
mxisd configuration keys can be added.

There's still the same default mxisd configuration. Keys of that default
are overwritten individually when added to `matrix_mxisd_extra_config`.