Fix configure get to support printing section values

[Dreamstick9]

Description of changes:

aws configure get previously returned an error when the requested
config key pointed to a section (dict) instead of a scalar value.
For example:

$ aws configure get s3  
varname (s3) must reference a value, not a section or sub-section.  

This PR fixes that by printing the section's key-value pairs in
config file format:

$ aws configure get s3  
signature_version = s3v4  

Changes:

• awscli/customizations/configure/get.py: Handle dict values in
  ConfigureGetCommand by printing each key-value pair instead of
  returning an error.
• tests/unit/customizations/configure/test_get.py: Updated
  test_get_section_returns_error to assert the correct output
  instead of the old error behavior.

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