Improve the _fix_plugin_schema.py

[matthieumarrast]

Yesterday we tried to update an mfbase plugin with objects declared on the wrong "public" schema.

While trying to execute the _fix_plugin_schema.py* script, we face the error:

$ _fix_plugin_schema.py plugin_<my_plugin> plugin_<my_plugin>
ALTER TABLE public.<table> SET SCHEMA plugin_<my_plugin>
Traceback (most recent call last):
  File "/opt/metwork-mfbase-2.2/bin/_fix_plugin_schema.py", line 33, in <module>
    cur.execute(sql2)
psycopg2.errors.InvalidSchemaName: schema "plugin_<my_plugin>" does not exist

*the script is missing in the mfbase version 2.2.7, we get it manually.

=> in case the schema does not already exist, the script must also execute:

CREATE SCHEMA plugin_<my_plugin> AUTHORIZATION plugin_<my_plugin>;
GRANT USAGE ON SCHEMA plugin_<my_plugin> TO plugin_<my_plugin>;

to be added to the script.