This updates the file backend to allow wildcard certificates to be

stored and fetched, and to use them as a fallback if a specific

certificate cannot be found.
Normal certs are now stored in the form of {hostname}__{port}.{ext},

and wildcard certs are stored as __.{hostname}__{port}.{ext}. This

avoids issues with naming a file with a wildcard, and _ is not a valid

character in domains so it's safe to use.
Wildcards are only permitted as the first character in the name.
When fetching a certificate, the backend checks first for an an exact

hostname match. If not found, it checks for a wildcard certificate. The

resulting Certificate object will use the specific hostname, but will

refer to the wildcard storage ID.
Wildcards are not supported nor needed for CA bundles and fingerprints.
There's also no special handling of certificates that list multiple

domains without using wildcards. In this case, administrators should

upload the cert for each hostname they need individually. We don't

expect this will come up much in real usage.

All unit tests pass.