Additional EBS volumes are re-attached when an instance is recreated

[BernhardBerbuir]

• support use case where #instances > #subnets
• add variable attached_block_device for configuring attached EBS volumes
• document conflict between inline and separate created EBS volumes
• inline ebs_block_device: use lookup for all attributes
• use Terraform 0.12 syntax everywhere

Description

This PR solves #116

For the implementation some design decisions have been required:

1. retrieving the correct AZ of an EBS volume.
   The implementation uses the subnets to determine the AZ instead of using calculated az information from aws_instance. This is necessary in order to prevent that the attached EBS volumes are destroyed and recreated when an EC2 instance is recreated.
2. sequence of creating EBS volumes: there are two possible strategies:
   1. support adding / removing instances
   2. support adding / removing attached EBS volumes
      I have decided to use the first solution because scaling the number of instances was more important for my use case. Adding additional attached EBS volumes might be possible when moving stuff in the statefile around (I did not test this)

[BernhardBerbuir]

OffTopic: would it be possible to use multi-line comments for variables and outputs? Especially for variables of type list(map(string)) a detailed description would be useful. Also using markdown could improve the documentation a lot.
Unfortunately I do not know if the generator would support this.

[bryantbiggs]

hi @BernhardBerbuir, thanks for the PR! I am a little confused by the pull request so please bare with me! what use case are you trying to solve with this pull request - right now, based on the comments it seems like we might be trying to solve too much in one change set

[BernhardBerbuir]

@bryantbiggs : this PR is no longer relevant for me, as I have created a permanent fork.
The idea of this PR was to solve several bugs and add features at once. It would have been better to create a separate PR for each problem.
If you have a question about a specific error / bug, then I'm happy to answer it.

[bryantbiggs]

thank you for the response @BernhardBerbuir - @antonbabenko we can close

[github-actions]

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