Add namelist_defaults_mosart.xml and adjust buildnml to read defaults.

[mvdebolskiy]

Right now default values for the namelist variables are hardcoded, this is not very good in case there will be defaults for different resolutions/symyears/etc/. It will also make the namelist generation consistent with what is there for CTSM.

[mvertens]

@mvdebolskiy - can you give an example of how you would want to see this.