default encoding for cmd steps configurable

[yaythomas]

Both the async and serial cmd/shell steps should initialise their default encoding from config.default_cmd_encoding.

config.default_cmd_encoding initialises from the env var DEFAULT_CMD_ENCODING. It defaults to None.

Async cmds will interpret this None as locale.getpreferredencoding(False).

For sync cmds can pass through None to the underlying subprocess API and the python runtime will take care of the using the default system locale.