virt-test: Bootstrap using other OS images.

The bootstrap procedure was hardcoded to use the JeOS image. If the
JeOS repository is unreachable the command will return an error. To use
a specific OS at bootstrap its name should be given in the run command
using the --guest-os parameter:
e.g. ./run -t qemu --bootstrap --guest-os=Enea.4.0.x86_64

Signed-off-by: Petre Pircalabu <petre.pircalabu@enea.com>
virt-test: virttest/standalone_test.py pep8 fixes

Signed-off-by: Petre Pircalabu <petre.pircalabu@enea.com>
virt-test: Use import defaults

Change 'from defaults use' with 'import defaults'

Signed-off-by: Petre Pircalabu <petre.pircalabu@enea.com>
virt-test: Download multiple assets on bootstrap

If the guest-os parameter matches multiple guest, the bootstrap script
should download all matching images.

Signed-off-by: Petre Pircalabu <petre.pircalabu@enea.com>
Merge pull request #1624 from pepi-enea/parameterized_bootstrap

Parameterized bootstrap