UUID from core backtrace if coredump is missing

Closes #882.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Only analyze vulnerabilities when coredump present

Also only generate core_backtrace if it's not already present.

Related to #882.

Signed-off-by: Martin Milata <mmilata@redhat.com>
Merge pull request #884 from abrt/gh882_corebt_uuid

Generate UUID from core backtrace if coredump is missing