[feat] Allow Syslog's address to be set from an environment variable

[vkarak]

• Also rename RFM_GRAYLOG_SERVER to RFM_GRAYLOG_ADDRESS for symmetry. RFM_GRAYLOG_SERVER is now deprecated.

Fixes #1056.

[codecov-commenter]

Codecov Report

Merging #1427 into master will decrease coverage by 0.01%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1427      +/-   ##
==========================================
- Coverage   91.84%   91.83%   -0.02%     
==========================================
  Files          82       82              
  Lines       12794    12798       +4     
==========================================
+ Hits        11751    11753       +2     
- Misses       1043     1045       +2     

Impacted Files	Coverage Δ
reframe/frontend/cli.py	78.52% <50.00%> (-0.36%)	⬇️

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[vkarak]

@bcfriesen This allows you to set the Syslog address through an environment variable. It's not exactly what's discussed in #1056, but I think it serves the same purpose. You can set this variable dynamically based on where ReFrame is run.