samples: dfu: pytest: reduce hardcoded global variables

[mniestroj]

Remove one hardcoded dfu project name in order to be able to quickly move (copy-paste) this test to other directory/sample.

Do not hardcode new firmware version (2.0.0) globally and move it into the test itself. This will allow to develop more DFU tests, that allow to test a flow with multiple rollouts (thus multiple versions).

Introduce a Firmware helper class, which will store both the path and the version of the firmware. Use imgtool package in order to figure out the version automatically from file name (in the context where only a firmware object is passed).

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