Allow Influxdb-reporter's send-interval to be modified

[Fahimfm]

Influxdb-reporter's send-interval can be modified from the settings.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.65%. Comparing base (f343e49) to head (a343b84).
⚠️ Report is 194 commits behind head on master.

Files with missing lines	Patch %	Lines
bzt/modules/influxdb_reporter.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1824      +/-   ##
==========================================
+ Coverage   89.61%   89.65%   +0.05%     
==========================================
  Files          70       70              
  Lines       19324    19341      +17     
==========================================
+ Hits        17315    17339      +24     
+ Misses       2009     2002       -7     

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