Fix syntax errors in Python erpcgen ouput

[peterhinson]

Fixes two issues in generated Python code related to structures containing unencapsulated unions. To better illustrate:

Given an IDL with this structure, union and discriminator:

enum TypeExample {
  DESC_FLT,
  DESC_U32
}

union UnionExample {
  case DESC_FLT:
    float flt
  case DESC_U32:
    uint32 u32
}

struct StructExample {
  UnionExample value @discriminator(kind);
  TypeExample kind
}

erpcgen will produce this somewhat broken Python code in common.py:

# Structures data types declarations
class StructExample(object):
    def __init__(self, kind=None):
        # syntax error! value not defined
        self.value = value # UnionExample
        self.kind = kind # TypeExample

    def _read(self, codec):
        # syntax error - common not defined (already in common module)
        self.value, self.kind = common.UnionExample()._read(codec) 
        return self
  # ...

Output after this PR:

# Structures data types declarations
class StructExample(object):
    def __init__(self, value=None, kind=None):
        self.value = value # UnionExample
        self.kind = kind # TypeExample

    def _read(self, codec):
        self.value, self.kind = UnionExample()._read(codec)
        return self
  # ...

[Hadatko]

These changes looks good to me. I improved/fixed tests for this case

[MichalPrincNXP]

Thank you for your effort!