tests: py: dfu: Provide functionality to set test and dummy files alt settings

After concatenation of "dfu_alt_info" variable from "dfu_alt_boot" and
"dfu_alt_system" it may happen that test and dummy files alt settings
are different than default 0 and 1.

This patch provides the ability to set different values for them.

Signed-off-by: Lukasz Majewski <l.majewski@samsung.com>
Acked-by: Stephen Warren <swarren@nvidia.com>
---
Changes for v3:
- replace variables declarations with ones read from configuration file
- remove not necessary str() conversion at DFU host command generation

Changes for v2:
- generate "alt_info" automatically
- use file names as alt settings instead of numerical values
- extend in-code documentation