Add support for sharing encrypted EBS resources cross account

[giovannt0]

Few changes:

• Added encrypted attribute to AWSVolume class.
• When calling CreateVolumeCopy() with a destination account that is not the same as the source account, we now check if the target volume is encrypted. If so, a one-time use KMS shared key (between source and destination account) is generated. Subsequently, the initial target volume is duplicated + encrypted with the temporary shared key. We then use this temporary volume for the remainder of the code (i.e., getting a new snapshot out of that temporary volume, sharing it with the destination account, and creating a copy within the destination account).

After execution, temporary volumes, snapshots and keys are deleted. The copy of the volume that sits in the destination account is encrypted with the default EBS/KMS key particular to that account.

Closes #63

Signed-off-by: Theo Giovanna gtheo@google.com

[codecov-io]

Codecov Report

Merging #70 into master will decrease coverage by 3.16%.
The diff coverage is 63.63%.

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
- Coverage   65.20%   62.03%   -3.17%     
==========================================
  Files           2        2              
  Lines         661      706      +45     
==========================================
+ Hits          431      438       +7     
- Misses        230      268      +38     

Flag	Coverage Δ
#nosetests	62.03% <63.63%> (-3.17%)	⬇️

Impacted Files	Coverage Δ
libcloudforensics/aws.py	67.94% <63.39%> (-9.43%)	⬇️
libcloudforensics/gcp.py	57.99% <65.00%> (-0.14%)	⬇️

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