#25786 Adds new parameter 'defer_keyword'

This adds a new manifest parameter called 'defer_keyword'. Here is the manifest description:

Advanced parameter. This allows for advanced customization around deferred folder creation. 
Deferred folder creation allows for the creation of partial subfolder structures depending
on a specific keyword (see main documentation for details). Before an app is launched, folders
are automatically created and by default (e.g. if you leave this setting as null), 
the launch app will pass the name of the engine as the deferred folder creation keyword. 
This makes it easy to set up deferred rules in your folder creation config for tk-maya, 
tk-nuke etc. However, if you for example wanted to set up specific deferred folder structures 
for Nuke and Nuke X (both running the nuke engine), you need a finer granularity. This setting
can then be used to override the default behaviour of just passing the engine name. Instead, you
can pass any string into the deferred folder creation (for example 'nuke' and 'nuke_x' in the case
above). This parameter is also useful if you want to use deferred folder creation in 
conjunction with launching of apps which do not have a toolkit engine defined - for these 
app launch instances, the engine setting is left blank, and therefore no engine name is passed
into the deferred folder creation. In such cases you can utilize this parameter to control
the deferred folder creation.


Is it worth it/possible to update the create_filesystem_structure api to be cleaner?  I'm wondering if it will be confusing further down the line?  This might not be a big deal though...