linting...
six.string_types
list-of-strings in examples
disallow extra_hosts support for list-of-dicts
A more thorough sets of tests for extra_hosts
Provide better examples
As per @aanand's [comment](https://github.com/docker/compose/pull/1158/files#r28326312)
I think it'd be better to check `if not isinstance(extra_hosts_line,
six.string_types)` and raise an error saying `extra_hosts_config must be
either a list of strings or a string->string mapping`. We shouldn't need
to do anything special with the list-of-dicts case.
order result to work with assert
use set() instead of sort()
Signed-off-by: CJ <lim@chernjie.com>
25ee3f0033