docs: Fix a few typos

[timgates42]

There are small typos in:

• cinderclient/shell.py
• cinderclient/tests/unit/test_shell.py
• cinderclient/utils.py

Fixes:

• Should read mutually rather than mutally.
• Should read identity rather than indentity.
• Should read authentication rather than authentification.

Semi-automated pull request generated by
https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[openstack-mirroring]

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