python: Add generic Key-Value parser.

Most of ofproto and dpif flows are based on key-value pairs. These
key-value pairs can be represented in several ways, eg: key:value,
key=value, key(value).

Add the following classes that allow parsing of key-value strings:
* KeyValue: holds a key-value pair
* KeyMetadata: holds some metadata associated with a KeyValue such as
  the original key and value strings and their position in the global
  string
* KVParser: is able to parse a string and extract it's key-value pairs
  as KeyValue instances. Before creating the KeyValue instance it tries
  to decode the value via the KVDecoders
* KVDecoders holds a number of decoders that KVParser can use to decode
  key-value pairs. It accepts a dictionary of keys and callables to
  allow users to specify what decoder (i.e: callable) to use for each
  key

Also, flake8 seems to be incorrectly reporting an error (E203) in:
"slice[index + offset : index + offset]" which is PEP8 compliant. So,
ignore this error.

Acked-by: Terry Wilson <twilson@redhat.com>
Acked-by: Eelco Chaudron <echaudro@redhat.com>
Signed-off-by: Adrian Moreno <amorenoz@redhat.com>
Signed-off-by: Ilya Maximets <i.maximets@ovn.org>

python/automake.mk

@@ -16,7 +16,6 @@ ovs_pyfiles = \
 	python/ovs/compat/sortedcontainers/sorteddict.py \
 	python/ovs/compat/sortedcontainers/sortedset.py \
 	python/ovs/daemon.py \
-	python/ovs/fcntl_win.py \
 	python/ovs/db/__init__.py \
 	python/ovs/db/custom_index.py \
 	python/ovs/db/data.py \
@@ -26,6 +25,10 @@ ovs_pyfiles = \
 	python/ovs/db/schema.py \
 	python/ovs/db/types.py \
 	python/ovs/fatal_signal.py \
+	python/ovs/fcntl_win.py \
+	python/ovs/flow/__init__.py \
+	python/ovs/flow/decoders.py \
+	python/ovs/flow/kv.py \
 	python/ovs/json.py \
 	python/ovs/jsonrpc.py \
 	python/ovs/ovsuuid.py \
@@ -42,6 +45,7 @@ ovs_pyfiles = \
 	python/ovs/version.py \
 	python/ovs/vlog.py \
 	python/ovs/winutils.py
+
 # These python files are used at build time but not runtime,
 # so they are not installed.
 EXTRA_DIST += \