Allow to KEEP configs unchanged on the environment

[vpodorozh]

Allow to KEEP configs unchanged on the environment => Base config will not override configs that are already set on environment, in case we use "!!KEEP" directive.

This is needed in stations when you have couple test environments, that share same test credentials. By utilizing this feature you could keep test credentials be stored under base scope, but on production scope you just add "!!KEEP" value to not override production configurations.
This would add a significant possibility to decrease configs duplications.

Example

Setup before changes

dev1:
- config/abc/key: "qwerty"

dev2:
- config/abc/key: "qwerty"

staging1:
- config/abc/key: "qwerty"

staging2:
- config/abc/key: "qwerty"

test_env1:
- config/abc/key: "qwerty"

test_env2:
- config/abc/key: "qwerty"

Setup after changes

base:
- config/abc/key: "qwerty"

production:
- config/abc/key: "!!KEEP"

[therouv]

@vpodorozh Thank you! Can you please fix the CS violations and also document the new behavior in the import documentation?

I'll take a look at the unit tests. That seems to be some other issue.

Thanks! Happy to merge after you've updated the PR 😊

[vpodorozh]

@vpodorozh Thank you! Can you please fix the CS violations and also document the new behavior in the import documentation?

I'll take a look at the unit tests. That seems to be some other issue.

Thanks! Happy to merge after you've updated the PR 😊

@therouv - done. Let me know in case smth needs to be fixed.
Thank you!