This is really a per-site setting, so it doesn't make sense to have
a role default. Anyways, HSTS is kinda tricky and potentially dang-
erous, so unless a vhost explicitly sets it to "yes" we shouldn't
enable it.
Note: also switch from using a boolean to using a string; it is st-
ill declarative, but at least now I don't have to guess whether it
is being treated as a bool or not.
Signed-off-by: Alan Orth <alan.orth@gmail.com>