(feat): Add fallback non-assigned block device mapping

[lasith-kg]

ebs-bootstrap should not fail if an Instance Store volume with an unassigned block device mapping is attached to the EC2 Instance. Especially if this Instance Store volume is not mentioned in the configuration file. Therefore, I have added a fallback clause where in if the Vendor Specific information fails to expose a block device mapping, it will now fallback to a safer option, rather than erroring out.

# Before
Vendor Specific: ephemeral0:none -> (Error) ❌

# After
Vendor Specific: ephemeral0:none -> /dev/ephemeral0

I've also updated the testing frame work to remove the need to explicitly mention what Vendor Specific Padding to use in the NVMe test cases