Support store authentication and authorization

[sgotti]

These are needed to let only authorized components to read and/or write to the cluster entries.

[sgotti]

This will have a sense when all the components will use only the store. So, at first, we have to remove the api handler in the sentinel (currently only used by stolonctl to update the clusterconfig). This will be done in #160.

In addition, since now we use libkv, it have to support setting store auth credentials.