Start adding support for CMIP6 SSP-RCP into cime

[ekluzek]

In order to support the new CMIP6 SSP-RCP scenarios in cime, we need the following updates into datm:

• CO2 for each scenario
• prescribed aerosols for each scenario
• CPLHIST forcing for each scenario

Those are all changes in cime/src/components/data_comps/datm/cime_config/config_component.xml

In addition

cime/src/drivers/mct/cime_config/config_component_cesm.xml needs the following:

Description added for SSP periods, and CCSM_BGC set for SSP periods.

[rljacob]

telecon: this is already in maint-5.6