cephadm: add unlink_file function

cephadm: add unlink_file function

Add the unlink_file function to file_utils. This fills in a gap between
python 3.6 and features provided in pathlib.Path in later versions of
python. Adds an option to ignore all errors for good measure.

Signed-off-by: John Mulligan <jmulligan@redhat.com>