Default to enforcing, as "[w]hile the US Standard region currently
allows non-compliant DNS bucket naming, we are moving to the same
DNS-compliant bucket naming convention for the US Standard region in the
coming months."
While we're at it, tighten the allowable characters:
* In DNS-compliant mode, only allow a-z, 0-9, hyphens, and periods.
* Otherwise, also allow A-Z and underscore.
Change-Id: Ida9b6c3260eb851b309c0f58f757e5376c66c0e0
eeb77b68f2