Config option constants naming

[ei-grad]

Follow up on #2040 (comment).

1. Config option name constants are named with the SECTION_XXX_ prefix. It seems more natural for me to use OPTION_ prefix for them.

2. STORAGEPATH and CREDENTIALPATH options name constant from SECTION_AWS section is assigned to SECTION_GCP and SECTION_LOCAL.

It looks that this is coming from the remainings of the legacy cloud config schema. And probably should be refactored.

What do you think?

[efiop]

Great points @ei-grad ! Btw, we could even consider dropping that legacy stuff, since it has been quite a while and we only had it in the first dvc versions, and were mentioned only by a very old tutorial. All our docs never mentionined legacy storagepath and friend at all.

[efiop]

No longer relevant with the new config.