Make the ssh key operation synchronous in tests

[Temikus]

Fixing test flakiness.

It looks like adding ssh keys has become slower since we didn't notice this before.

[codecov]

Codecov Report

Merging #488 into master will increase coverage by 6.51%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
+ Coverage   81.41%   87.92%   +6.51%     
==========================================
  Files         339      339              
  Lines        5639     6809    +1170     
==========================================
+ Hits         4591     5987    +1396     
+ Misses       1048      822     -226

Impacted Files	Coverage Δ
lib/fog/compute/google/models/disk_types.rb	90.9% <0%> (ø)	⬆️
lib/fog/compute/google/models/machine_types.rb	86.95% <0%> (ø)	⬆️
lib/fog/compute/google/models/servers.rb	96.22% <0%> (ø)	⬆️
lib/fog/google/requests/sql/insert_user.rb	85.18% <0%> (-14.82%)	⬇️
lib/fog/compute/google/models/zones.rb	100% <0%> (ø)	⬆️
lib/fog/compute/google/models/regions.rb	100% <0%> (ø)	⬆️
...ib/fog/storage/google_json/requests/copy_object.rb	100% <0%> (ø)	⬆️
...fog/storage/google_json/requests/get_bucket_acl.rb	100% <0%> (ø)	⬆️
...b/fog/storage/google_json/requests/list_buckets.rb	100% <0%> (ø)	⬆️
lib/fog/google/requests/sql/delete_ssl_cert.rb	81.13% <0%> (-18.87%)	⬇️
... and 123 more

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