Function 'remove_lvm_physical_volume' passing invalid input

[lmlg]

Hello,
The function charmhelpers.contrib.storage.linux.lvm.remove_lvm_physical_volume is passing an erroneous argument to the method communicate of the standard Popen class.

Here: https://github.com/juju/charm-helpers/blob/master/charmhelpers/contrib/storage/linux/lvm.py#L63

we can see that it's being passed a string, but after using it in the cinder-lvm charm, I get the following traceback:

  File "lib/charm/openstack/cinder_lvm.py", line 265, in prepare_volume
    clean_storage(device)
  File "lib/charm/openstack/cinder_lvm.py", line 293, in clean_storage
    remove_lvm_physical_volume(block_device)
  File "/var/lib/juju/agents/unit-cinder-lvm-0/.venv/lib/python3.6/site-packages/charmhelpers/contrib/storage/linux/lvm.py", line 63, in remove_lvm_physical_volume
    p.communicate(input='y\n')
  File "/usr/lib/python3.6/subprocess.py", line 848, in communicate
    self._stdin_write(input)
  File "/usr/lib/python3.6/subprocess.py", line 801, in _stdin_write
    self.stdin.write(input)
TypeError: a bytes-like object is required, not 'str'

A simple fix would be to use the bytes equivalent: b'y\n'.