MINOR: stktable: support optional index for array types in {set, clear, show} table...

MINOR: stktable: support optional index for array types in {set, clear, show} table commands

As discussed in GH #2286, {set, clear, show} table commands were unable
to deal with array types such as gpt, because they handled such types as
a non-array types, thus only the first entry (ie: gpt[0]) was considered.

In this patch we add an extra logic around array-types handling so that
it is possible to specify an array index right after the type, like this:

  set table peer/table key mykey data.gpt[2] value
  # where 2 is the entry index that we want to access

If no index is specified, then it implicitly defaults to 0 to mimic
previous behavior.