Use the standard Serverless servicePath in the plugin

[Frank Schmid]

This is a Feature Proposal

Description

Until now the plugin overwrites the servicePath property in Serverless during its lifecycle and
sets it back in the cleanup lifecycle event. This makes the whole system very fragile and dependent on
the consistent use of the property in Serverless as well as all other plugins. In the extreme, we can
throw the dice, if the next Serverless version will lead to problems or not.

This behavior should be changed and the plugin should use the given servicePath as plugins are
expected to do.

I think the change is not trivial, because there has to be a reason, why this approach has been taken.
We should analyze every single step in the plugin and check if there are any issues that can arise from
changing it to a stable use.

After the implementation has been done, it should be tested properly and ripen in master for a while.