Do not try to upgrade option if not needed.

It fixes the following exception, when starting without any pre-existing configuration file:
```
  File "./picard/config_upgrade.py", line 106, in upgrade_to_v1_3_0_dev_3
    _s["preferred_release_countries"].split("  ")
AttributeError: 'list' object has no attribute 'split'
```
Moved info about the change to function's doc

As discussed with @mwiencek, we don't need to expose those to average user,
nor to mark those for translations.
Shorten lines using intermediate variables
Output hooks docstrings to log.debug

Example of output:
```
D: 10:57:46 Config upgrade 1.3.0dev1 -> 1.3.0dev2: Option "preserved_tags" is now using comma instead of spaces as tag separator (PICARD-536)
```
Merge pull request #1 from zas/listoptions

Improve hooks documentation and debug output, fix few issues