Adds algorithm to the pages domain key and remote mirror credentials encrypted attributes for forward compatibility with attr_encrypted 3.0.0.

aes-256-cbc is the default algorithm for attr_encrypted 1.x, but the default is changed in 3.0 and thus must be declared explicitly.

See https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4216/ for more information. This will prevent OpenSSL errors once the code from that MR is merged into EE.