of_config: add OFPortStateType class

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Reviewed-by: YAMAMOTO Takashi <yamamoto@valinux.co.jp>
ofconfig: add commit and discard_changes operation support

They are necessary for switches that don't support the feature of
modifying 'running' configuration directly; instead, needs to modify
'candidate' configuration and then commit (or discard).

Also fix do_get method's comment typo.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>