[google compute] Add license strings for new GCE images

[erjohnso]

[google compute] Allow adding license strings to GCE images

Description

Allow for insertion of license strings when creating a new GCE image [https://cloud.google.com/compute/docs/reference/latest/images]

Status

First pass at implementation, needs tests.

Checklist (tick everything that applies)

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes)

/cc @katrielt

[pquentin]

Looks good! Can you please update test_ex_create_image too? Thanks!

[erjohnso]

Thank you @pquentin - tests added and passing, I'll go ahead and merge.

[codecov-io]

Codecov Report

Merging #1136 into trunk will increase coverage by <.01%.
The diff coverage is 78.57%.

@@            Coverage Diff             @@
##            trunk    #1136      +/-   ##
==========================================
+ Coverage   85.47%   85.47%   +<.01%     
==========================================
  Files         348      348              
  Lines       66421    66443      +22     
  Branches     5909     5914       +5     
==========================================
+ Hits        56774    56794      +20     
- Misses       7240     7241       +1     
- Partials     2407     2408       +1

Impacted Files	Coverage Δ
libcloud/test/compute/test_gce.py	97.56% <100%> (ø)	⬆️
libcloud/compute/drivers/gce.py	76.04% <25%> (-0.08%)	⬇️
libcloud/test/compute/test_ec2.py	97.77% <0%> (-0.14%)	⬇️
libcloud/compute/drivers/ec2.py	74.25% <0%> (+0.19%)	⬆️

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[pquentin]

Great!