[RM-11694] Add optional directory to write_file()

[RM-11694] Add optional directory to write_file()

This is mostly for testing purposes.  The method has no way to
specify a directory independently from the file name, which it
makes it difficult to use with automated tests.  It will get
refactored more later, but for now add a new kwarg that pass
in a dir to write to.  Since a lot of callers are passing in
absolute paths (.e.g. "/etc/ceph/keyring"), we look for the
beginning slash and chop it off so that we can use os.path.join()
and write into something like /tmp/etc/ceph.

Signed-off-by: Travis Rhoden <trhoden@redhat.com>