Protect against version downgrade in secrets tune command

[catsby]

Fixed #5237 by guarding against removing the version option in secrets backends.

I'm not 100% confident in the placement of the test, I struggled constructing one in logical_system_test.go where maybe it makes more sense to be.

The test is currently in command/secrets_tune_test.go, I was able to construct a test that reproduces the regression and verifies it's fixed by 27f008b .

[calvn]

This is semi-related since you're working with the options param:

vault/http/sys_mount_test.go

Line 701 in fb89af7

"options": map[string]interface{}{"test": "true"},

and

vault/http/sys_mount_test.go

Line 706 in fb89af7

"options": map[string]interface{}{"test": "true"},

should be removed since where's testing for the absence of the values after it's being unset. The current logic doesn't let you unset the options map once that's set since we are only writing to storage if len(options) > 0.

We should either remove that part if we don't support unsetting, or change it so that we can unset (but still preserve options["version"] when doing a complete unset to prevent downgrade). However, this would sort of revert #4325.

[catsby]

Closing in favor of #5809