python SamDB: init default flags to FLG_DONT_CREATE_DB

python SamDB: init default flags to FLG_DONT_CREATE_DB

The current defaults for SamDB are to create the database file if it does not
exist.  Most of the uses of SamDB assume the database already exists, and so
auto-creation is not the desired behaviour.

TDB will overwrite an existing non TDB file with a newly created TDB file.
This becomes an issue when using alternate database file formats i.e. lmdb.

Signed-off-by: Gary Lockyer <gary@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>