release-ec2: bugfixes

1. It turned out that Amazon changes standard AMIs from time to time,
   AMI ID changes as well in this case. As a result Windows AMI we
   used as a template for OSv releases became anavailable.
   The script will use earlier OSv release AMI instead to avoid
   this class of problems.

2. Invalid strings compare syntax fixed.

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
release-ec2: beautifications

1. Use multithreaded build when possible
2. Suppress output on bucket removal

Signed-off-by: Dmitry Fleytman <dmitry@daynix.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>