reana.yaml: executing shell commands in commands

[tiborsimko]

Users of Serial workflow engine may want to execute commands in the step command.

Using $(...) does not validate:

commands:
          - g++ -g -std=c++11 -O3 -Wall -Wextra -Wpedantic -o skim skim.cxx $(root-config --cflags --libs)

Using backticks works fine:

commands:
          - g++ -g -std=c++11 -O3 -Wall -Wextra -Wpedantic -o skim skim.cxx `root-config --cflags --libs`

Since the preferred shell syntax to execute commands is $(command arg1 arg2) instead of <backquote>command arg1 arg2<backquote>, we should investigate allowing the former as well.

P.S. Observed for the Htt example https://github.com/cms-opendata-analyses/HiggsTauTauNanoAODOutreachAnalysis/pull/2/files