Add `/usr/sbin` to the PATH.
On Fedora zuul machines `/usr/sbin` seems to not be in
the PATH by default, which makes fdisk to not be found
making some docker tests to fail.
Change-Id: I9732e45dee43a53f5ecd4d3fd4853d82f236c288
Signed-off-by: Salvador Fuentes <salvador.fuentes@intel.com>