Automatically generated dynamic config contains default values for unmodified params

When certain maxctrl commands are executed under maxctrl, MaxScale will silently create a dynamic config file under /var/lib/maxscale.cnf.d to accommodate them.

When doing so, it will add there all available params (which were not modified by the maxctrl command) with using their default values. This makes any modification of these default values via the static config impossible (because they will be overwritten by the dynamic config) and also quite confusing (because MaxScale will refuse to use values, set explicitly in the static file, for which we have not explicitly configured any dynamic value).

Example:

• Start MaxScale with vanilla config.
• Run "maxctrl enable log-priority info" to change the log level.
• Now check /var/lib/maxscale/maxscale.cnf.d/maxscale.cnf which has tons of other values, including "admin_host" with its default value, 127.0.0.1.
• Try enabling the MaxScale GUI in the static config file by setting "admin_host" to 0.0.0.0. It will not work, because the dynamic config has the default value for this param explicitly set. Same for any other MaxScale config param.

Expected behaviour: MaxScale should only write to dynamic config file parameters that we have changed explicitly using maxctrl.