I get the following error message:
Error: Failed to open /opt/logs/logstash/logstash.log for writing: Permission denied - /opt/logs/logstash/logstash.log

What user does Logstash run as and what's the output of the following command?

ls -l /opt/logs/logstash/logstash.log /opt/logs/logstash

Is this the correct forum or should I file a bug-report?

This is a good place for questions. Let's wait with the bug report until we know there actually is a bug. Bug reports are read by fewer people so you have a better chance of getting help here.

Again, what user owns and what are the permissions of /opt/logs/logstash and /opt/logs/logstash/logstash.log?

Same problem here.

ubuntu@ip-10-0-0-113:/var/log/logstash$ cat logstash.stdout
Error: Failed to open /var/log/logstash/logstash.log for writing: No such file or directory - /var/log/logstash/logstash.log

This is often a permissions issue, or the wrong
path was specified?
You may be interested in the '--configtest' flag which you can
use to validate logstash's configuration before you choose
to restart a running system.

ubuntu@ip-10-0-0-113:/var/log/logstash$ ls -l /var/log/logstash/logstash.log /var/log/logstash
-rw-r--rw- 1 logstash logstash 0 Feb 3 06:59 /var/log/logstash/logstash.log

total 28
-rw-r--r-- 1 root root 0 Feb 3 22:32 logstash.err
-rw-r--rw- 1 logstash logstash 0 Feb 3 06:59 logstash.log
-rw-r--r-- 1 root root 346 Feb 3 22:32 logstash.stdout

ubuntu@ip-10-0-0-113:/var/log/logstash$ sudo service logstash configtest
Configuration OK

ubuntu@ip-10-0-0-113:/opt/logstash$ cat Gemfile
source "https://rubygems.org"
gem "logstash-core", "2.1.1"