Fixes#1780
Specifically fixes the calls to `os.symlink` within the `move_data_directory` function, as used to update the symlink(s) for any WAL or tablespace directories following an init failure. The new name for a WAL and/or tablespace directory is now passed in as in as the first argument when calling `os.symlink`, while the second argument is now the name of the symlink that is being recreated.
This aligns with the Python documentation for `os.symlink`, which states the following regarding the first two arguments for the `os.symlink` function (arguments `src` and `dst`):
> Create a symbolic link pointing to src named dst (https://docs.python.org/3/library/os.html#os.symlink)
With this change, all WAL and/or tablespaces directories will be properly renamed following a init failure, and the `FileExistsError` that would otherwise occur when attempting to recreate the associated symlinks is no longer thrown, e.g.:
```bash
2020-12-05 20:35:42.219 GMT [262] LOG: unrecognized configuration parameter "invalid" in file "/pgdata/mycluster1/postgresql.auto.conf" line 5
2020-12-05 20:35:42.219 GMT [262] FATAL: configuration file "/pgdata/mycluster1/postgresql.auto.conf" contains errors
2020-12-05 20:35:43,220 ERROR: postmaster is not running
2020-12-05 20:35:43,223 INFO: removing initialize key after failed attempt to bootstrap the cluster
2020-12-05 20:35:43,292 INFO: renaming user defined tablespace directory and updating symlink: /tablespaces/ts1/ts1
2020-12-05 20:35:43,314 INFO: renaming data directory to /pgdata/mycluster1_2020-12-05-20-35-43
Traceback (most recent call last):
```
Additionally, any/all WAL and/or tablespace symlinks are properly recreated, e.g.:
```bash
$ ls -l pg_tblspc
total 0
lrwxrwxrwx. 1 postgres postgres 40 Dec 5 20:35 16414 -> /tablespaces/ts1/ts1_2020-12-05-20-35-43
```
