[TASK] Use StringFrontend for HashService cache
[TASK] Use StringFrontend for HashService cache

The use of StringFrontend was intended, but not configured in
Caches.yaml, so a VariableFrontend was used instead for the persistent
cache.

Change-Id: I3270dcaa43eff4b9a01ac046dc7d1092b48df2b7
Releases: master, 3.0
Original-Commit-Hash: 46472338ad879d302f1ac07878b45f4f94ce1567

gerhard-boden/flow-development-collection,bwaidelich/flow-development-collection,dfeyer/flow-development-collection,gerhard-boden/flow-development-collection,daniellienert/flow-development-collection,dlubitz/flow-development-collection,gerhard-boden/flow-development-collection,neos/flow-development-collection,fcool/flow-development-collection,kitsunet/flow-development-collection,neos/flow-development-collection,kdambekalns/flow-development-collection,daniellienert/flow-development-collection,robertlemke/flow-development-collection,bwaidelich/flow-development-collection,chewbakartik/flow-development-collection,albe/flow-development-collection,kdambekalns/flow-development-collection,albe/flow-development-collection,neos/flow-development-collection,robertlemke/flow-development-collection,kdambekalns/flow-development-collection,robertlemke/flow-development-collection,fcool/flow-development-collection,fcool/flow-development-collection,mkeitsch/flow-development-collection,daniellienert/flow-development-collection,chewbakartik/flow-development-collection,aertmann/flow-development-collection,fcool/flow-development-collection,mkeitsch/flow-development-collection,kitsunet/flow-development-collection,aertmann/flow-development-collection,neos/flow-development-collection,kdambekalns/flow-development-collection,chewbakartik/flow-development-collection,dfeyer/flow-development-collection,dfeyer/flow-development-collection,robertlemke/flow-development-collection,dlubitz/flow-development-collection,dlubitz/flow-development-collection,mkeitsch/flow-development-collection,albe/flow-development-collection,bwaidelich/flow-development-collection,kitsunet/flow-development-collection,aertmann/flow-development-collection,dfeyer/flow-development-collection