Add virt-builder source

Add implementation for virt-builder source which aims to create
container root file system from VM image build with virt-builder.

Usage examples:
    $ virt-bootstrap virt-builder://fedora-25 /tmp/foo
    $ virt-bootstrap virt-builder://ubuntu-16.04 /tmp/bar --root-password secret
    $ virt-bootstrap virt-builder://fedora-25 /tmp/foo -f qcow2 --idmap 0:1000:10
    $ sudo virt-bootstrap virt-builder://fedora-25 /tmp/foo --idmap 0:1000:10

Tests are also introduced along with the implementation. They cover
creation of root file system and UID/GID mapping for 'dir' and 'qcow2'
output format by mocking the build_image() method to avoid the time
consuming call to virt-builder which might also require network
connection with function which creates dummy disk image.
Setting root password is handled by virt-builder and hence the
introduced test only ensures that the password string is passed
correctly.

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