Add stable-2.12 to CI

[Andersson007]

SUMMARY

Add stable-2.12 to CI

stable-2.12 has been recently branched

If it's relevant, would be also nice to have this added to community.mongodb, etc.

[codecov[bot]]

Codecov Report

Merging #173 (74a9e97) into master (88c8cdd) will decrease coverage by 0.48%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #173      +/-   ##
==========================================
- Coverage   78.37%   77.89%   -0.49%     
==========================================
  Files          25       25              
  Lines        1850     1850              
  Branches      431      431              
==========================================
- Hits         1450     1441       -9     
- Misses        205      210       +5     
- Partials      195      199       +4     

Impacted Files	Coverage Δ
.../cassandra/plugins/modules/cassandra_stopdaemon.py	82.05% <0.00%> (-10.26%)	:arrow_down:
...andra/plugins/modules/cassandra_upgradesstables.py	86.48% <0.00%> (-2.71%)	:arrow_down:
...unity/cassandra/plugins/modules/cassandra_table.py	80.88% <0.00%> (-1.48%)	:arrow_down:
...ty/cassandra/plugins/modules/cassandra_keyspace.py	79.45% <0.00%> (-1.37%)	:arrow_down:

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[rhysmeister]

codecov jobs are duped for some reason (expect it's nothing to do with the change).

[rhysmeister]

codecov issue unrelated to change- Merging.