Store the render file directory as stagingDir

[BigRoy]

Changelog Description

Store the render file directory as stagingDir

Additional review information

Fix #194

Testing notes:

1. Test with PR Store in instance data whether the staging dir set is a custom one ayon-core#1066
2. With ValidateRenderImageRule plug-in enabled publishing renders should work fine.
3. Submitting renders should work as intended with both custom staging dir set and when disabled.

[LiborBatek]

I have tested both without any custom staging directory on and with my custom one...in both cases no errors shown and my DL jobs finnished ok and renders got properly integrated into AYON.

LGTM!

and finally when used with staging template:

[LiborBatek]

...also checked if custom staging dir was used and properly cleaned up....all good!