[ML] Remove mention of models in inference actions

[davidkyle]

This renames the GET, PUT and DELETE inference APIs removing the model parts.

• inference.delete_model renamed to inference.delete
• inference.get_model renamed to inference.get
• inference.put renamed to inference.put

The GET response now has a endpoints field instead of models

#previously
{
    "models": [...]  
}

#with this change
{
    "endpoints": [...]  
}

And error messages are change to refer to inference endpoints rather than models

[elasticsearchmachine]

Pinging @elastic/ml-core (Team:ML)

[davidkyle]

@elasticmachine update branch

[pquentin]

Can't review the Elasticsearch code itself, but left a couple copywriting suggestions.

[maxhniebergall]

LGTM

[davidkyle]

@elasticmachine update branch

[pquentin]

Thanks! LGTM.