Make Gaussian initial std configurable about tensorforce

[michaelschaarschmidt]

Further issue:

Another thing that might be nice to add to the library is the ability to query the action without any exploration noise. This could come, for example, in the form of an exploration=True keyword in agent.act(). In the case where the policy is stochastic this could boil down to evaluating the mean model, which would require some minor internal changes.

[AlexKuhnle]

Both integrated.