suite/build_matrix: get rid of '.yaml' in descriptions

suite/build_matrix: get rid of '.yaml' in descriptions

Test descriptions have structured representation
fragments the test config is composed from.
It's worth noticing that no other files except yaml
are used for description composition.
Although having the .yaml extentions in the description
looks redundant and has no use in the code.

This change removes many '.yaml' sufixes from
descriptions which make it more compact and easy
to read.

Signed-off-by: Kyr Shatskyy <kyrylo.shatskyy@suse.com>