Add "host_name_separator" variable to Context class

Add "host_name_separator" variable to Context class

This feature will provide Kubernetes context the ability to handle the
context name inside the class itself, providing to the developer an
abstraction of the possible naming limitations in Kubernetes. E.g.:
"dot" character in Pod names is no allowed [1].

[1] https://github.com/fabric8io/fluent-plugin-kubernetes_metadata_filter/issues/58

JIRA: YARDSTICK-1188

Change-Id: I82121f970b550170357a443b6340be7900602a57
Signed-off-by: Rodolfo Alonso Hernandez <rodolfo.alonso.hernandez@intel.com>