rename: tarantool-sys branch to picodata-submodule

This allows to operate submodule with the one-liner:

```
git submodule update --remote tarantool-sys/
```

This patch also updates the submodule in a mentioned way.

.gitmodules

@@ -4,6 +4,7 @@
 [submodule "tarantool-sys"]
 	path = tarantool-sys
 	url = ../../tarantool.git
+	branch = picodata-submodule
 	ignore = dirty
 [submodule "vshard"]
 	path = vshard

tarantool-sys @ 783c6c81

-Subproject commit f7cb4a9c314ef94882279e24c59b91a9dfaaf1b2
+Subproject commit 783c6c81bb66c9a062efca2eb8b05a084ba37978