Do not require specifying quota when specifying location watermarks

[p4vook]

No description provided.

[p4vook]

@l0kix2 could you please check this PR out? It still says that 1 approval is required.

[l0kix2]

@l0kix2 could you please check this PR out? It still says that 1 approval is required.

sure, sorry for the delay, will look into it

[l0kix2]

It looks a bit hard to understand and error-prone with all these nested ifs.
I suggest we rewrite it in less nested way:

// setting quota
if quota != nil {
    storeLocation.Quota = *quota
}

// setting low watermark
if location.LowWatermark != nil {
    storeLocation.LowWatermark = location.LowWatermark.Value()
} else if quota != nil {
    gb := float64(1024 * 1024 * 1024)
    storeLocation.LowWatermark = int64(math.Min(0.1*float64(storeLocation.Quota), float64(25)*gb))
}

// setting other watermarks if low watermark is set
if storeLocation.LowWatermark > 0 {
    storeLocation.HighWatermark = storeLocation.LowWatermark / 2
    storeLocation.DisableWritesWatermark = storeLocation.HighWatermark / 2
    storeLocation.TrashCleanupWatermark = storeLocation.LowWatermark
}

[l0kix2]

Also I suggest to add test cases for scenarios which aren't currently covered, so this code won't become broken in the next changes.
Tests for config generator are rather easy to write: just add some test cases with specs having quota=nil + low_watermark=nil, quota!=nil + low_watermark=nil, quota=nil + low_watermark!=nil
and simply check that canonized result looks as expected in config.