[astute] false positive result of reboot for reprovisioning task fails provisioning

This is the actual reason for https://bugs.launchpad.net/fuel/+bug/1616925 (since it was fixed in a different scope let's keep it as is, meanwhile this new bug is still valid)

Detailed bug description:
 For some unknown reasons soft_reboot for provisioned nodes could fail and trigger false positive results, so astute didn't know that reboot was never occurred, but it kept reprovisioning task to go further. In result, provisioning will fail later with the following error message:

 flock: provision: No such file or directory

Possibly affected code area: https://github.com/openstack/fuel-astute/blob/stable/mitaka/lib/astute/provision.rb#L536-L543

Steps to reproduce:
 Happens from time to time. Could be tricky to reproduce. No known exact steps for reproducing.

Expected results:
 Astute shouldn't continue to provision nodes which aren't loaded into bootstrap image. Perhaps, it should retry to reboot nodes failed to boot to bootstrap or abort provisioning earlier.
Actual results:
 Astute runs provisioning further until it fails due to missing 'provision' executable script.

Additional information:
For more information please take a look to comments and attachments from #21 till #25 of original bug https://bugs.launchpad.net/fuel/+bug/1616925