bindep does not work on Alpine linux

[Ehud Kaldor]

bindep's Depends module does not parse the Alpine distro name, causing it to fail:

/swift # bindep --profiles
Platform profiles:
platform:alpine
platform:alpine-3
platform:alpine-3.7
platform:alpine-3.7.0

Configuration profiles:
platform:centos
platform:dpkg
platform:fedora
platform:redhat
platform:rpm
platform:suse
test

/swift # bindep test
Traceback (most recent call last):
  File "/usr/bin/bindep", line 10, in <module>
    sys.exit(main())
  File "/usr/lib/python2.7/site-packages/bindep/main.py", line 73, in main
    errors = depends.check_rules(rules)
  File "/usr/lib/python2.7/site-packages/bindep/depends.py", line 258, in check_rules
    installed = self.platform.get_pkg_version(rule[0])
AttributeError: 'Depends' object has no attribute 'platform'
/swift #