nova: Show stdout/stderr when "openstack flavor list" fails

[toabctl]

When running into the problem that the "openstack flavor list" command
fails, we want to know the error and not only see a comment that the
nova api might be down.
This is useful for debugging.

(cherry picked from commit b8776b6)

[skazi0]

@toabctl please add the orignal PR link to the description

[toabctl]

Ok. Now we see the real failure:

10:52:02 [2019-05-20T08:51:49+00:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
10:52:02 [2019-05-20T08:51:49+00:00] FATAL: RuntimeError: ruby_block[Get current flavors] (nova::flavors line 107) had an error: RuntimeError: Flavor list not obtained, is the nova-api down?
10:52:02 
10:52:02 Certificate did not match expected hostname: public.cluster-data.vm2.cloud.suse.de. Certificate: {'subjectAltName': [('DNS', 'cluster-data.vm2.cloud.suse.de'), ('DNS', 'public-cluster-data.vm2.cloud.suse.de')], 'subject': ((('commonName', u'cluster-data.vm2.cloud.suse.de'),),)}
10:52:02 Unable to establish connection to https://public.cluster-data.vm2.cloud.suse.de:8774/v2.1/a914f5a21c23499a967f1464a43e2183/flavors/detail: HTTPSConnectionPool(host='public.cluster-data.vm2.cloud.suse.de', port=8774): Max retries exceeded with url: /v2.1/a914f5a21c23499a967f1464a43e2183/flavors/detail (Caused by SSLError(CertificateError("hostname 'public.cluster-data.vm2.cloud.suse.de' doesn't match either of 'cluster-data.vm2.cloud.suse.de', 'public-cluster-data.vm2.cloud.suse.de'",),))
10:52:02