mapserv auto-discovery does not work

[Turbo87]

Context

I'm trying to use the following mapserver source:

sources:
  mwp_mapserver_bin:
    type: mapserver
    supported_srs: ['EPSG:3857']
    req:
      layers: MWP
      map: foo.map
      transparent: true
    image:
      format: image/png
    mapserver:
      working_dir: ../mapserver

according to the documentation and 069fb44 the mapserver.binary setting is now optional and should automatically find the binary from PATH.

Instead of that I'm seeing the following warning:

mapproxy.config - WARNING - Missing mapserver binary for source 'mwp_mapserver_bin'

Expected Behavior

mapproxy should find the mapserv binary automatically

Actual Behavior

An configuration validator warning is displayed

Possible Fix

My guess is that https://github.com/mapproxy/mapproxy/blob/1.11.0/mapproxy/config/validator.py#L136-L165 needs to be adjusted to account for the new code introduced in 069fb44

Steps to Reproduce

see above

Context

see above

Your Environment

• Version used: 1.11.0
• Environment name and version (e.g. Python 2.7.5 with mod_wsgi 4.5.9): Python 2.7.13
• Operating System and version: Debian Stretch