Allow multiple args values for call and instantiate commands

Sergej Kostjucenko requested to merge aj/fix-args-multiple-values into master

Created by: ascjones

After the migration from structopt to clap derive #457, the default for Vec opts is for multiple_occurrences, e.g. --args arg1 --args arg2 whereas with structopt it was multiple_values e.g. --args arg1 arg2. This resulted in an error like Found argument '2' which wasn't expected, or isn't valid in this context when providing multiple values.

As noted in docs for multiple_values, there are issues with its use so we should consider changing how contract arguments are specified.

However in the meantime this reverts to the original behaviour with structopt of accepting multiple values.

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