[#214] Make tezos-client data dir configurable

Description

Problem: We should run different nettest in different tezos-client environments in order to avoid unexpected errors (e.g. in case when we run multiple nettest scenarios simultaneously).

Solution: Add -d/--data-dir option which specifies path to tezos-client data directory that is used when calling tezos-client.

Related issue(s)

Resolves #214 (closed)

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