Add configurable default profile

[fierlion]

Summary

Update to allow for a configurable alternate credential profile other than default.
#3241

Implementation details

Add a new env var, ECS_ALTERNATE_CREDENTIAL_PROFILE, to allow for overriding the default credential profile.

Testing

built agent with existing logging in place. updated /etc/ecs/ecs.config to add

ECS_ALTERNATE_CREDENTIAL_PROFILE="testProfile"

checked info level logs for the override message.

New tests cover the changes: yes

Description for the changelog

Licensing

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[Realmonia]

nit: sorry for my poor English but can you help me understand why you choose alternate over alternative? Thanks!

Edit: nvm I kinda understand it - the idea you want to convey here is it is not another optional but an override. Not sure if there are better word options to avoid the confusion that this is going to be used as a backup profile - considering it is not a back up but an override.

[fierlion]

nit: sorry for my poor English but can you help me understand why you choose alternate over alternative? Thanks!

They're essentially synonyms.

'alternate is frequently used to mean "offering or expressing a choice." There are established uses for both that we regularly encounter: a director's cut of a film may feature an alternate ending; a traffic update warns motorists to seek an alternate route.
Many examples, though, use the word simply to mean "offering a choice apart from what is traditional or expected'

https://www.merriam-webster.com/words-at-play/differences-between-alternate-and-alternative