ceph-disk: add --prepend-to-path to control execution

ceph-disk: add --prepend-to-path to control execution

/usr/bin is hardcoded in front of some ceph programs which makes it
impossible to control where they are located via the PATH.

The hardcoded path cannot be removed altogether because it will most
likely lead to unexpected and difficult to diagnose problems for
existing installations where the PATH finds the program elsewhere.

The --prepend-to-path flag is added and defaults to /usr/bin : it prepends
to the PATH environment variable. The hardcoded path is removed
and the PATH will be used: since /usr/bin is searched first, the
legacy behavior will not change.

Signed-off-by: Loic Dachary <loic@dachary.org>