qa/cephfs: change default timeout from 900 secs to 300

qa/cephfs: change default timeout from 900 secs to 300

15 minutes is unnecessarily large as a default value for timeout for a
command. Not having to wait unnecessarily on a crash of a command will
reduce teuthology's testing queue and will save individual developer's
time while running tests locally.

Whatever lines are modified for this purpose are also modified to follow
the stlye guideline, specfically wrapping at 80 characters.

Fixes: https://tracker.ceph.com/issues/54236
Signed-off-by: Rishabh Dave <ridave@redhat.com>