BIGTOP-3015: hadoop-spark bundle correction

[jamesbeedy]

This change removes the "hadoop-plugin" charm from the "hadoop-spark" bundles and tests as it isn't needed in the bundle.

Fixes: https://issues.apache.org/jira/browse/BIGTOP-3015

[kwmonroe]

@jamesbeedy, the reason hadoop-client was in this bundle was to act as an endpoint for people that were accustomed to running jobs and managing hdfs from there. iow, muscle memory and scripts that do this will work on any of the hadoop-x bundles::

juju run --unit hadoop-client/0 'hdfs dfs -cmd'

It was also there in case someone started with hadoop-spark, switched the spark runtime from YARN to HA, then scaled spark to x units. In that scenario, the single hadoop-client would be a recognizable endpoint to facilitate hdfs administration, versus making users do that on a random spark unit.

That said, I'm coming around to your proposal to remove hadoop-client. Hulk smashing apps on a single unit has never been recommended and is only done to balance app density with resource cost. As you already know, spark is a hadoop-client from the charms perspective, so an additional explicit client isn't needed.

I'm +1 to this removal if you'll also pull out the Client-related statements from the bundle README.md.