managers/ec2: split instance removal and instance termination

Related to #25.
feaas/runners: make code more generic

Provide a method for initializing the multi locker. Every runner so far
does need the multi locker, it makes no sense to always initialize it in
the runner's init method.
runners/instance_starter: use generic parameters
runners: add instance_terminator

Related to #25.
managers/ec2: do not remove instance in the terminate_instance call

The instance terminator will do that for us.

Closes #25.