add the cluster_name option to CrateLayer

so one can build a cluster of testlayer instances
update crate server for tests to 0.43.1
fix tests
document cluster_name option in CrateLayer test
add configurable host to CrateLayer
add information about starting a cluster using CrateLayer
Merge pull request #102 from crate/m/cluster_layer

add the cluster_name option to CrateLayer