read agama kernel params if available (bsc#1234678)

[jreidinger]

Problem

Agama when proposing bootloader configuration getting all agama related parameters without any filtering.

bsc 1234678

Solution

Use specific agama cmdline file that contain only kernel related arguments without any agama specific ones.
It needs also agama-project/agama#1896 . But it does not crash without it, just will again fallback to /proc/cmdline.

Testing

• Added a new unit test
• Tested manually

[coveralls]

coverage: 41.879% (-0.01%) from 41.889%
when pulling 2c8bd91 on read_agama_dir
into 037b94b on master.