BF: DataladOrchestrator: Set DATALAD_SSH_IDENTITYFILE in __init__

[kyleam]

DataladOrchestrator.prepare_remote() sets DATALAD_SSH_IDENTITYFILE so that DataLad can work with SSH targets, in particular those for our AwsEc2 resource, that need a custom key file that's not specified in ssh_config. Doing this in prepare_remote(), however, is problematic because prepare_remote() isn't necessarily called before other methods like fetch() that also require DATALAD_SSH_IDENTITYFILE to be configured. From the command line, this prevents fetching a job with reproman jobs for a resource that depends on a custom key.

Move the DATALAD_SSH_IDENTITYFILE handling to init() so that it is also in effect for resurrected orchestrators.

Fixes #561.

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Given the current SSH setup for our tests, I haven't come up with a straightforward way to test this.

@chaselgrove If you have time, could you verify that this fixes the issue you reported?

[chaselgrove]

Fixes #561

[codecov[bot]]

Codecov Report

Merging #562 (bde5c93) into master (b880459) will not change coverage. The diff coverage is 25.00%.

@@           Coverage Diff           @@
##           master     #562   +/-   ##
=======================================
  Coverage   89.64%   89.64%           
=======================================
  Files         149      149           
  Lines       12943    12943           
=======================================
  Hits        11603    11603           
  Misses       1340     1340           

Impacted Files	Coverage Δ
reproman/support/jobs/orchestrators.py	94.17% <25.00%> (ø)

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[yarikoptic]

Thank you!