server: add an endpoint for updating an ensemble's node templates

[aszs]

Add an api endpoint that patches the "ResourceTemplate" json that is exported

It should match the patch endpoints we already are using in our graphql api which we've implemented twice already:

js: https://github.com/onecommons/unfurl-gui/blob/handover/apollo-server/resolvers.js#L152

ruby: https://github.com/onecommons/gitlab-oc/blob/handover/oc/lib/oc/unfurl_deploy_meta.rb#L50

let's add one in Python :)

[aszs]

basic PR (#147) has been merged but I just realized it probably doesn't invalidate the in memory cache used by the other endpoints... Moving to sprint backlog for now...