(PUP-1840) Respect digest algorithm in filebucket file

Original code implemented by Jared Jennings <jared.jennings.ctr@us.af.mil>
(PUP-1840) Respect digest algorithm in filebucket dipper

Original code implemented by Jared Jennings <jared.jennings.ctr@us.af.mil>
(PUP-1840) Use configured digest_algorithm as default checksum_type for file resources

Without this patch, file resources will always try to checksum
themselves using MD5. On FIPS 140-2 compliant hosts, this will
fail. This patch adds sha256 as a permissible value for the File
resource's checksum parameter, and makes the checksum parameter
default to using the digest_algorithm, as set in the puppet.conf.
(PUP-1840) Use single location for default digest algorithm