Set default port for proxy client target

[pkittenis]

Resolves #248

[codecov]

Codecov Report

Merging #250 (cc57e33) into master (ff6498f) will decrease coverage by 0.34%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
- Coverage   93.98%   93.63%   -0.35%     
==========================================
  Files          17       17              
  Lines        1413     1414       +1     
==========================================
- Hits         1328     1324       -4     
- Misses         85       90       +5     

Impacted Files	Coverage Δ
pssh/clients/native/single.py	93.73% <100.00%> (-1.30%)	⬇️

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