CLEANUP: stick-table/cli: take the "show table" context definition out of the appctx

CLEANUP: stick-table/cli: take the "show table" context definition out of the appctx

This makes use of the generic command context allocation so that the
appctx doesn't have to declare a specific one anymore. The context is
created during parsing. The code also uses st2 which deserves being
addressed in separate commit.