[FEATURE] custom map: Keep descriptions of builtin arguments

[zstadler]

Is your feature request related to a problem? Please describe.
When the value of a builtin argument is set by a custom map YAML file, the description is not shown in the log.

For example, when using the original value and description of tmpdir, the log file says:

0:00:00 DEB - argument: tmpdir=data/tmp (temp directory)

When changing the value of tmpdir,

args:
  tmpdir: "/tmp"

the log file says:

0:00:00 DEB - argument: tmpdir=/tmp (no description provided)

It is also possible to set any description for the built argument. For example,

args:
  tmpdir: 
    description: Toronto Motorsports Park direction
    default: "/tmp"

will produce this log line:

0:00:00 DEB - argument: tmpdir=/tmp (Toronto Motorsports Park direction)

Describe the solution you'd like
I'd like the description of builtin arguments to be kept regardless of existence or non-existence of a description: key in their override in the custom map YAML.

In both examples above, I would expect the log file to say:

0:00:00 DEB - argument: tmpdir=/tmp (temp directory)