chore(build): put --coverage CXX_FLAGS for TESTCOV build
chore(travis): use consistent build name

 - have build name is in the form of
       "<BRANCH>_<CXX_COMPILER>_<BUILD_TYPE>(_TC)?"
   to have consistency over time.
 - have "Generator" field in cdash XMLs which is in the form of
        "<GIT_COMMIT>###<GIT_SUBJECT>"
   it will be displayed as a small mouseover-text.
chore(travis): push all linux builds to cdash