Add a DOI XML variable

[bertinia]

Starting with CESM2.0, there will be a DOI pointing to the associated CESM model page URL.
This DOI variable will be included in every component history output file as a global metadata attribute. It needs to be set as a CESM XML variable in:

cime/driver_cpl/cime_config/config_component_cesm.xml.

and then picked up by every component model output writer routine similar to the
global attribute time_period_freq.

This is for CMIP6 compliance and also provides additional provenance information in the history file global metadata.

[bertinia]

This issue was moved to ESMCI#1327