Make sure that an arn including 'role' won't pass

[matthieudelaro]

Currently, an ARN such as the following 'arn:aws:iam::424242424242:user/matthieu_role' is accepted as a valid ARN by the get_execution_role() function, despite the fact that a test has been implemented:

    if 'role' in arn:
        return arn
    message = 'The current AWS identity is not a role: {}, therefore it cannot be used as a SageMaker execution role'
    raise ValueError(message.format(arn))

Unfortunately, the test passes if ever the arn includes the term 'role'.

This commit improves this test by testing that 'role' is between a : and a /, in order to match arn like arn:aws:iam::424242424242:role/sagemaker_role_matthieu

[mvsusp]

Thanks for contributing @matthieudelaro.
Can you add a unit test on

sagemaker-python-sdk/tests/unit/test_session.py

Line 31 in 157d867

def test_get_execution_role():

with the missing use?

[mvsusp]

Please add unit tests.

[codecov-io]

Codecov Report

Merging #65 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #65   +/-   ##
=======================================
  Coverage   92.79%   92.79%           
=======================================
  Files          71       71           
  Lines        5373     5373           
=======================================
  Hits         4986     4986           
  Misses        387      387

Impacted Files	Coverage Δ
src/sagemaker/session.py	83.42% <100%> (ø)	⬆️

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[laurenyu]

continued this PR in #570

[matthieudelaro]

@laurenyu, thanks a lot for all you work that made the merging possible!