consistent file location handling across APIs

[vnitinv]

What does this PR do?

Fix Juniper#146

What issues does this PR fix or reference?

Fixes:

• all API should have the same behavior for the user-provided file.
• In case of proxy model, we can have file located either on master or proxy server itself. So we should handle both cases.

Previous Behavior

• install_os, load, install_config took file from master, file_copy get_table used to take file from proxy.
• Also if master/proxy running at same machine, install_os would unnecessarily copy huge file to same server to some tmp file.

New Behavior

• if master/proxy running at same allocation, file is not copied and can use local file location
• All APIs now can take file from master using salt://

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltstack.com/en/master/topics/development/changelog.html
• [ y ] Tests written/updated

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Yes/No

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[vnitinv]

@dmurphy18 Would you help in reviewing this?