Rename the updateIfExist option

[scottinet]

Currently, the Collection.createDocument method allows the updateIfExist option. Which does a document replacement if it exists in Kuzzle's database.

Since this option is misleading, and because we may want to add a createOrUpdate API route later, we propose the following:

• remove the updateIfExist option
• add a ifExist option, taking the following values: error (default), replace