Add max_header_size to swift.conf-sample and relative UT

1. Add explanation of MAX_HEADER_SIZE into swift.conf-sample as same as
other settings in swift.conf. Especially point out the default size of
header line in eventlet is 8192 which is the main reason why we set 8192
for MAX_HEADER_SIZE in swift.

2. Add some unit tests to check valid settings in swift.conf. Test cases
in test_constraints use /etc/swift/swift.conf if exists, and if any
wrong settings are in it (MAX_META_VALE > MAX_META_OVERALL_SIZE), swift's
unit test must fail. These new unit tests is used in this case.

Change-Id: I7bb21951d46050163c1b7bceac8d49302b9209f7
Merge "Add max_header_size to swift.conf-sample and relative UT"