add some 'osd pool default' config variables

Added the following variables to the [global] section:
- osd pool default pg num
- osd pool default pgp num
- osd pool default min size
- osd pool default crush rule

These variables are only set if a value was set, they are optional
and not mandatory.

Signed-off-by: Danny Al-Gaaf <danny.al-gaaf@bisect.de>