node.xml: add primary keys to all the tables
node.xml: make the os column of appliances and enum

now all the os columns are of type enum
commands: remove hack for bug in mysql-python when using sets

The fix to this bug was introduced in bfe712f16aa66f894bf2c5d9fee51cbb97ac9938
and it is now not necessary since we don't use anymore sets
and the os column is an enum
node.xml: adding an index to the name column of nodes