sdcm.cluster: Refactor graph generation and separate latency graphs
sdcm.cluster: Refactor graph generation and separate latency graphs

Fixes #122.

Separate latency graphs, instead of plotting latency data
to a single plot. While we're at it, refactor the graph
generation procedure, to reduce code duplication and
increase readability.

Signed-off-by: Lucas Meneghel Rodrigues <8ad3460456640ab580599f5440c5a8d925a620b9@scylladb.com>

scylladb/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,scylladb/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-longevity-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,amoskong/scylla-cluster-tests,scylladb/scylla-cluster-tests