Add LG_SSH_CONNECT_TIMEOUT environment variable

[JoshuaWatt]

Adds an environment variable called LG_SSH_CONNECT_TIMEOUT which controls the timeout passed to SSH when attempting a connection.

Checklist

• [x] Documentation for the feature
• [ ] Tests for the feature

• [ ] The arguments and description in doc/configuration.rst have been updated

• [ ] Add a section on how to use the feature to doc/usage.rst

• [ ] Add a section on how to use the feature to doc/development.rst

• [x] CHANGES.rst has been updated

• [x] PR has been tested

• [x] Man pages have been regenerated

[codecov[bot]]

Codecov Report

Base: 63.3% // Head: 63.3% // Increases project coverage by +0.0% :tada:

Coverage data is based on head (bda0b39) compared to base (b116c16). Patch coverage: 100.0% of modified lines in pull request are covered.

Additional details and impacted files

@@          Coverage Diff           @@
##           master    #958   +/-   ##
======================================
  Coverage    63.3%   63.3%           
======================================
  Files         152     152           
  Lines       11303   11309    +6     
======================================
+ Hits         7163    7169    +6     
  Misses       4140    4140           

Impacted Files	Coverage Δ
labgrid/driver/sshdriver.py	58.0% <100.0%> (+0.1%)	:arrow_up:
labgrid/util/agentwrapper.py	96.8% <100.0%> (+<0.1%)	:arrow_up:
labgrid/util/ssh.py	91.4% <100.0%> (+<0.1%)	:arrow_up:

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