Avoid having to update PreReleaseBrandingLabel for each release

[mmitche]

The branding update code should only have to update the pre-release iteration and pre-release version label. The aspnetcore specific branding label (PreReleaseBrandingLabel) should be calculated off of this. It should be calculated in the following way:

pre-release version label == 'alpha' -> "Alpha $(PreReleaseVersionIteration)"
pre-release version label == 'preview' -> "Preview $(PreReleaseVersionIteration)"
pre-release version label == 'rc' -> "RC $(PreReleaseVersionIteration)"
pre-release version label == 'rtm' -> "RTM"
pre-release version label == 'servicing' -> "Servicing"