Fixes #6842: Refreshes all metadata and datasource

[prashant-shahi]

Fixes for #6842. I, myself, encountered this issue when one of the druid clusters IPs was wrong, I was not able to refresh metadata or load up the list of latest datasources.

Added logic to not throw any exceptions but set a valid_cluster boolean to just flash a warning for the invalid or irresponsive clusters.

[codecov-io]

Codecov Report

Merging #6944 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master    #6944      +/-   ##
==========================================
- Coverage   63.98%   63.97%   -0.01%     
==========================================
  Files         422      422              
  Lines       20537    20540       +3     
  Branches     2230     2230              
==========================================
  Hits        13141    13141              
- Misses       7264     7267       +3     
  Partials      132      132

Impacted Files	Coverage Δ
superset/connectors/druid/views.py	66.21% <0%> (-1.37%)	⬇️

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