Add a HEALTHCHECK

[timdorr]

Fixes #167

I swiped the script from @knordstrom and @Lukkie. The general gist is that it asks Zookeeper for the brokers it knows about and checks that it is in that list. So, a little better than just a port check with netcat.

If anyone has ideas for improvement, please let me know and I'll add it to this PR.

[sscaling]

The response in my comment to this issues still applies - #167 (comment)

Health checks don't really provide much use other than an a queryable label on the running container.

It'd be great to understand what real-world use-case this particular solution solves to know on whether to proceed or not.

As an aside - i'm not sure this is a great approach to health check, all it does is validate that the broker has connected to zookeeper and been assigned a znode - it has no real indication of whether Kafka is healthy or not, the only half decent way of doing so is via a successful query to the brokers. Even with that said, if your brokers are in a distinct network from your clients and communication is via an external port a health check from inside the broker network becomes misleading, i.e. it could appear healthy but be completely unusable.

[fvigotti]

In my installation which may be slightly different but probably not so much.. the
shell commands ( ie : zookeeper-shell.sh ) use same env variables as kafka..
with the provided script there could be problems ie: if the pod is limited in memory to something less than 200% of the -Xmx or if the kafka is exporting JMX maybe would be usefull to reset/reimport from another name those variables

KAFKA_HEAP_OPTS="-Xms200M -Xmx500M"
## avoid conflicts on jmx ports with kafka 
KAFKA_JMX_OPTS=""
JMX_PORT=""

[jayvdb]

trailing whitespace on this line and the previous line

also fwiw, using "health☑️" as a branch name makes it difficult for others to use your branch.