not possible to promote proxy configuration to staging through API

It is not possible to promote proxy configuration to staging environment because endpoint for promoting requires name of environment from which it is promoted, see

Proxy Config Promote
/admin/api/services/{service_id}/proxy/configs/{environment}/{version}/promote.json POST 

There is workaround: update proxy configuration by

/admin/api/services/{service_id}/proxy.xml  PATCH

and probably this will be fixed in this PR:
https://github.com/3scale/porta/pull/1326#discussion_r336540612

Dev Notes

• Deprecate the behaviour in 2.7: on proxy update, a proxy config is automatically created. It will not be the case anymore in 2.8. customer should use the promote endpoint introduced in 2.7
• remove the behaviour in 2.8