Add minReplicaCount support for scaledjob

[FrittenToni]

Proposal

Keda scaledjob should be able to scale to minReplicaCount by default to ensure that there is always a specified amount of jobs/pods available to pick up work immediately.

Use-Case

For our project it would be helpful if Keda could scale to minReplicaCount by default in order to avoid pod bootstrapping time.
The feature could be used in combination with "custom" strategy and "customScalingQueueLengthDeduction" property.

Without "minReplicaCount" and "custom" strategy only the following scaling can be achieved:

With "minReplicaCount" and "custom" strategy we can achieve the following result:

Anything else?

PR has been created already.