mke2fs: do not use full path

At the moment, tempest assumes that mke2fs will reside at /usr/sbin.
This is not necessarily true, for which reason it's safer to just
avoid using the full path.

Change-Id: I6455bbc2e5cec14d2520b19fc00d183d02879e4b
diff --git a/tempest/config.py b/tempest/config.py
index c50ebbe..ec7807f 100644
--- a/tempest/config.py
+++ b/tempest/config.py
@@ -806,7 +806,7 @@
                default="password",
                help="Password used to authenticate to an instance."),
     cfg.StrOpt('ssh_shell_prologue',
-               default="set -eu -o pipefail; PATH=$$PATH:/sbin;",
+               default="set -eu -o pipefail; PATH=$$PATH:/sbin:/usr/sbin;",
                help="Shell fragments to use before executing a command "
                     "when sshing to a guest."),
     cfg.IntOpt('ping_size',