Support for update operation to fuelclient added

What is done:
- New method `update` that represents flag for `fuel env` command added
  to `actions.Environment` object;
- all needed mappings to include new operation into parsing system added
  into `argument.py` file;
- tests updated;

Change-Id: Ibfc35d01a19d4848b84201197b912169848e3419
Closes-Bug: #1330910