Use vm.detect_package_manager() to check if apk is available

[glehmann]

This PR is part of a stack containing 2 PRs:

1. master
2. "Use vm.detect_package_manager() to check if apk is available" (this PR)
3. Move root partition resizing logic to VM.grow_root_partition() #542

[Wescoeur]

Suggested change

	elif self.file_exists('/usr/bin/apt-get'):
	return PackageManagerEnum.APT_GET
	elif self.file_exists('/sbin/apk'):
	return PackageManagerEnum.APK
	else:
	return PackageManagerEnum.UNKNOWN
	if self.file_exists('/usr/bin/apt-get'):
	return PackageManagerEnum.APT_GET
	if self.file_exists('/sbin/apk'):
	return PackageManagerEnum.APK
	return PackageManagerEnum.UNKNOWN

[glehmann]

One line shorter, I like it :)
Done.

[glehmann]

Test image: