Add GIT_SUBMODULE_DEPTH variable

As an alternative to !2218 (closed)

Extending !2107 (merged) by applying changes as discussed in !2107 (comment 362856579) and !2218 (comment 828303750)

What does this MR do?

Introduce a GIT_SUBMODULE_DEPTH CI variable which configures the git submodule depth, when not set the GIT_DEPTH is used.

Why was this MR needed?

To have the ability to split GIT_DEPTH in GIT_DEPTH and GIT_SUBMODULE_DEPTH so one could have some more fine-tuning options.

What's the best way to test this MR?

What are the relevant issue numbers?

Closes !2218 (closed)