no longer require SHA to upload artifacts to log

Since the verification of a signature will, by definition, include
verifying the content has not been altered, it is unnecessary to require
users of the CLI or REST API to specify the SHA256 hash of the content
when creating a new entry into the log.

Note that the server will still compute the hash and store it in the log
for ease of comparison.

Fixes #180

Signed-off-by: Bob Callaway <bcallawa@redhat.com>