Bring back clm5-cam6 init_interp_attributes for ne0CONUS grid

[slevis-lmwg]

Description of changes

Same as title, plus bring back the corresponding tests in the ctsm_sci test-suite.

Specific notes

Contributors other than yourself, if any:
@ekluzek

CTSM Issues Fixed (include github issue #):
Resolves #2544

Are answers expected to change (and if so in what way)?
No

Any User Interface Changes (namelist or namelist defaults changes)?
Added clm5-cam6 init_interp_attributes to namelist_defaults_ctsm.xml

Does this create a need to change or add documentation? Did you do so?
No

Testing performed, if any:
As posted in #2544 the next four tests PASS with this PR's code mods:

SMS_Ln9.ne0POLARCAPne30x4_ne0POLARCAPne30x4_mt12.IHistClm50Sp.derecho_intel.clm-clm50cam6LndTuningMode_1979Start--clm-nofireemis
SMS_Ln9.ne0CONUSne30x8_ne0CONUSne30x8_mt12.IHistClm50Sp.derecho_intel.clm-clm50cam6LndTuningMode_2013Start--clm-nofireemis
SMS_Ln9.ne0ARCTICne30x4_ne0ARCTICne30x4_mt12.IHistClm50Sp.derecho_intel.clm-clm50cam6LndTuningMode_1979Start--clm-nofireemis
SMS_Ln9.ne0ARCTICGRISne30x8_ne0ARCTICGRISne30x8_mt12.IHistClm50Sp.derecho_intel.clm-clm50cam6LndTuningMode_1979Start--clm-nofireemis

[slevis-lmwg]

PASS ./build-namelist_test.pl