securedrop-admin: rename validate into load and return content
securedrop-admin: rename validate into load and return content

Since validation is done by loading the YAML file, we may as well use
the same function for both purposes.

conorsch/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,conorsch/securedrop,heartsucker/securedrop,heartsucker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ehartsuyker/securedrop,heartsucker/securedrop,conorsch/securedrop,ehartsuyker/securedrop,conorsch/securedrop,ehartsuyker/securedrop