Properly cache zookeeper connection strings

[nmuesch]

What does this PR do?

Properly uses the last_zk seen cache for the case when users provided a list of connection strings. Before we were assuming there was only one.

Also adds the exception to an error message we provide.

Motivation

We accept both a string and a list of strings for zk_connect_str in the yaml file. However, the _should_zk function was attempting to use this to access a dictionary. In the case of it being a list, you'd end up with an exception about list being an unhashable type. This addresses that, with a test :)

Additional Notes

Anything else we should know when reviewing?

Review checklist (to be filled by reviewers)

• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
• PR must have changelog/ and integration/ labels attached
• Feature or bugfix must have tests
• Git history must be clean
• If PR adds a configuration option, it must be added to the configuration file.

[codecov]

Codecov Report

Merging #3333 into master will decrease coverage by 5.37%.
The diff coverage is 87.5%.

@@            Coverage Diff            @@
##           master   #3333      +/-   ##
=========================================
- Coverage   85.97%   80.6%   -5.38%     
=========================================
  Files         700       8     -692     
  Lines       36908     531   -36377     
  Branches     4453     113    -4340     
=========================================
- Hits        31731     428   -31303     
+ Misses       3956      72    -3884     
+ Partials     1221      31    -1190

[zippolyte]

💜