app: fix boolean handling in argparse module

There was a complaint that tarantoolctl --show-system option is
very hard to use. It incorrectly parsed passed values, and
provided strange errors.

    tarantoolctl cat --show-system true
    Bad input for parameter "show-system". Expected boolean, got "true"

    tarantoolctl cat --show-system 1
    Bad input for parameter "show-system". Expected boolean, got "1"

    tarantoolctl cat --show-system=true
    Bad input for parameter "show-system". Expected boolean, got "true"

First of all, appeared that the complaining people didn't read
documentation in 'tarantoolctl --help'. It explicitly says, that
'--show-system' should go after a file name, and does not have a value.

Secondly, even having taken the documentation into account, the
errors indeed look ridiculous. 'Expected boolean, got "true"'
looks especially weird.

The problem appeared to be with argparse module, how it parses
boolean parameters, and how stores parameter values not specified
in a command line.

All parameters were parsed into a dictionary: parameter name ->
value. If a name is alone (no value), then it is boolean true.
Otherwise it was always a string value. An attempt to specify
an explicit parameter value 'true' led to storing string 'true'
in that dictionary.

Consequential check for boolean parameters was trivial:
type(value) == 'boolean', which was obviously wrong, and didn't
pass for 'true' string, but passed for an empty value.

Closes #4076

(cherry picked from commit 03f85d4c)