[#321] Use '--mode mockup' in tezos-client calls

Description

Problem: 'tezos-client' has '--mode' option and it's possible to use 'mock' node using which 'tezos-client' doesn't perform any actual RPC requests to the node from the config. Using such mode can speed up nettests and reduce amount of unexpected RPC failures that can be caused by 'tezos-client'.

Solution: Add '--mode ' to the 'tezos-client' call arguments and use mockup mode for all calls that don't perform any actions on the real chain and only use data that is stored in the 'tezos-client' directory (e.g. keys and addresses).

Related issue(s)

Resolves #321 (closed)

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