Move user instantiation to doExecute

[lezzago]

Issue #, if available:

Description of changes:
Move user variable instantiation to doExecute to prevent user object being shared between 2 users logged into the same cluster because of race conditions

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

Merging #343 (c1f6d7d) into master (039ece9) will decrease coverage by 0.26%.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##             master     #343      +/-   ##
============================================
- Coverage     79.56%   79.29%   -0.27%     
  Complexity      202      202              
============================================
  Files           151      151              
  Lines          5324     5323       -1     
  Branches        701      698       -3     
============================================
- Hits           4236     4221      -15     
- Misses          715      729      +14     
  Partials        373      373              

Impacted Files	Coverage Δ	Complexity Δ
...ting/transport/TransportDeleteDestinationAction.kt	32.69% <100.00%> (ø)	0.00 <0.00> (ø)
...alerting/transport/TransportDeleteMonitorAction.kt	40.00% <100.00%> (ø)	0.00 <0.00> (ø)
...lerting/transport/TransportExecuteMonitorAction.kt	68.42% <100.00%> (ø)	0.00 <0.00> (ø)
...rch/alerting/transport/TransportGetAlertsAction.kt	75.75% <100.00%> (ø)	0.00 <0.00> (ø)
...erting/transport/TransportGetDestinationsAction.kt	76.38% <100.00%> (ø)	0.00 <0.00> (ø)
...ch/alerting/transport/TransportGetMonitorAction.kt	80.00% <100.00%> (ø)	0.00 <0.00> (ø)
...rting/transport/TransportIndexDestinationAction.kt	61.19% <100.00%> (ø)	0.00 <0.00> (ø)
.../alerting/transport/TransportIndexMonitorAction.kt	59.66% <100.00%> (+0.32%)	0.00 <0.00> (ø)
...alerting/transport/TransportSearchMonitorAction.kt	58.33% <100.00%> (ø)	0.00 <0.00> (ø)
...roforelasticsearch/alerting/alerts/AlertIndices.kt	65.64% <0.00%> (-10.69%)	0.00% <0.00%> (ø%)
... and 2 more

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