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.

Related-Prod: PRODX-28634

Change-Id: I6455bbc2e5cec14d2520b19fc00d183d02879e4b
3 files changed