Support multiple security group IDs

[ConstantinoSchillebeeckx]

Unless I'm missing something it doesn't look like this supports multiple security group IDs when starting instances. Is that right? If so, would it be possible to add that feature?

[jpalomaki]

Currently that is true, you can only specify a single security group.

I believe this issue could also be solved with #65 (by allowing a user to specify an EC2 launch template that configures several security groups).

[hajapy]

Could also be as simple as: https://github.com/hajapy/ec2-github-runner/commit/de9824c96b4fb54e7a7ac113463bec541ffc2935