Modify input YAML for FV3 increment converter (LETKF)

[DavidNew-NOAA]

This PR is a companion to Global Workflow PR #2593 and GDASApp PR #1104, which adds the "atmensanlfv3inc" job to Global Workflow. That job converts the JEDI ensemble increments into FV3 readable increments.

This PR modifies the the input YAML for fv3jedi_fv3inc_lgetkf.yaml.j2 in two ways:

1. It reads FMS restart backgrounds, instead of cubed sphere history backgrounds, since reading cubed sphere history backgrounds will be implemented later.
2. It (temporarily for now) uses FV3-JEDI restart fields metadata files for reading JEDI increments, instead of history fields metadata files, until we can start staging backgrounds as cubed sphere history files for LETKF and thus can start using history fields metadata files in the LETKF input YAML. The reason for this temporary hack is that the JEDI LETKF application uses the same fields metadata for both reading background and writing increments.