Validate ddev config flag input values

[andrewfrench]

The Issue
In #1092, support for defining config values can through flags passed into ddev config was added, but these values are currently naively written to config.yaml without any validation. There are several instances where providing wrong values might lead to cryptic error messages at the container or application level, but we can do basic validation on the values before accepting and writing them.

The Solution
Validate values passed into ddev config flags before writing config.yaml. In some cases, like PHP versions, one source of truth should be defined to indicate which values are supported and valid config values. In other cases, like FQDNs and hostnames, basic string validation can be used.