Relabel /tftpboot if selinux is used

nova-baremetal install creates this directory. We call restorecon
on /tftpboot so that it gets the correct label of tftpdir_t.

It it is not relabeled, baremetal instances will fail to boot
because images cannot be read if selinux is in enforcing mode.

Change-Id: Id03a68a10c4379c7ba5546508dde1b874260b2d1
Merge "Relabel /tftpboot if selinux is used"