Clarify allas boto3 instructions

[msainio]

Proposed changes

Updated instructions on configuring credentials. Reworked content at start of page. Closes #2142.

Preview page: https://csc-guide-preview.rahtiapp.fi/origin/clarify-allas-boto3-instructions/data/Allas/using_allas/python_boto3/

Checklist before requesting a review

• [x] I have followed the instructions in the Contributing and Styleguide documents.
• [x] My pull request passes the automated tests. (If not, visit pull requests at Travis CI and have a look at the job log.)
• [x] I have included a link to the appropriate preview page (select your branch from the list).