config: check schema before applying modifications

[shcheklein]

Version:

WARNING: Unable to detect supported link types, as cache directory '.dvc/cache' doesn't exist. It is usually auto-created by commands such as `dvc add/fetch/pull/run/import`, but you could create it manually to enable this check.
DVC version: 0.81.3+7f5e4a
Python version: 3.7.5
Platform: Darwin-18.2.0-x86_64-i386-64bit
Binary: False
Package: None

Reproduce:

dvc remote add test https://example.com
dvc remote modify test random_key
dvc version

will produce: ERROR: configuration error - config file error: extra keys not allowed

Probably we should do a check before we set the value.