Collecting backtraces as artifacts was added.

Now, when `dnet_ioserv` process failed to stop, backtraces for all threads will
be collected as test artifacts.
pgrep was replaced with pidof:

Using `pidof` will be more correctly: `pgrep` returns pid based on name; and
`pidof` returns pid of runned `/usr/bin/dnet_ioserv`.
collecting-artifacts playbook was corrected
Merge pull request #75 from kis8ya/features/artifacts-backtraces

Collecting backtraces as artifacts was added.