[WIP] add new method to set embargo

[peetucket]

fixes #481

• Confirm if new rights (after embargo) passed in via method parameter are simply any one of the allowed rights ("world", "stanford", etc.) as enumerated here: https://github.com/sul-dlss/dor-services/blob/master/lib/dor/datastreams/rights_metadata_ds.rb#L54-L67 or do we need more info passed in?
• Confirm if we need to set embargoReleaseDate in rightsMetadata on the read node only, or if we also need an additional parameter for the discover node
• If we need discover node, add additional parameters to allow it to be specified
• generally review to make sure the logic makes sense and conforms to https://consul.stanford.edu/display/chimera/Embargo+management+--+the+embargoMetadata+datastream

Once method is available in dor-services gem, we can go back and implement the setting of embargo in argo (sul-dlss/argo#1051) and then possibly update both etd and hydrus to use the new method instead of implementing their own ways of setting embargo.

[coveralls]

Coverage increased (+0.04%) to 92.027% when pulling e095afb on set-embargo into 7757946 on master.