syslog adapter: add support for some template functions

[nailgun]

This can be useful for extracting structured data from container name. For example, Kubernetes name containers in format k8s_CONTAINER_POD_NAMESPACE_UID_0 so I can now extract structured data by defining this var:

SYSLOG_STRUCTURED_DATA='kube container="{{ index (split .ContainerName "_") 1 }}" pod="{{ index (split .ContainerName "_") 2 }}" namespace="{{ index (split .ContainerName "_") 3}}"'

[michaelshobbs]

tests fail. probably needs to be rebased with master