gnatsd config should specify defaults

[shilov]

Documentation for the gnatsd config should specify default values for properties that are required.

Alternatively, the example config can be updated to reflect the defaults for required properties.

This is a common practice in ops. It's not instinctive to visit the gnatsd godoc to figure out what the defaults are.

[mcqueary]

@shilov would it be sufficient to list the defaults in the gnatsd usage statement (i.e. the output of gnatsd -h)? Some of the defaults are already defined there, but not all.

[shilov]

@mcqueary that would definitely be helpful