Checks for the existance of an AWS credential file.
        If the environment variable AWS_CREDENTIAL_FILE is
        set and points to a file, that file will be read and
        will be searched credentials.
        Note that if credentials have been explicitelypassed
        into the class constructor, those values always take
        precedence.

def check_for_credential_file(self):
        """
        Checks for the existance of an AWS credential file.
        If the environment variable AWS_CREDENTIAL_FILE is
        set and points to a file, that file will be read and
        will be searched credentials.
        Note that if credentials have been explicitelypassed
        into the class constructor, those values always take
        precedence.
        """
        if 'AWS_CREDENTIAL_FILE' in os.environ:
            path = os.environ['AWS_CREDENTIAL_FILE']
            path = os.path.expanduser(path)
            path = os.path.expandvars(path)
            if os.path.isfile(path):
                fp = open(path)
                lines = fp.readlines()
                fp.close()
                for line in lines:
                    if line[0] != '#':
                        if '=' in line:
                            name, value = line.split('=', 1)
                            if name.strip() == 'AWSAccessKeyId':
                                if 'aws_access_key_id' not in self.args:
                                    value = value.strip()
                                    self.args['aws_access_key_id'] = value
                            elif name.strip() == 'AWSSecretKey':
                                if 'aws_secret_access_key' not in self.args:
                                    value = value.strip()
                                    self.args['aws_secret_access_key'] = value
            else:
                print 'Warning: unable to read AWS_CREDENTIAL_FILE'