LU-13270 tests: dom-performance fixes

LU-13270 tests: dom-performance fixes

Patch makes the fs cleanup at the start of dom-performance
optional and depending on t-f global DO_CLEANUP option
value. For DO_CLEANUP=false script removes only the
directories created by previous dom-performance session.
Without this fix test run 2nd time fail with:
  lfs setdirstripe: dirstripe error on '/mnt/testfs/dp_dne':
      stripe already set
  lfs setdirstripe: cannot create stripe dir '/mnt/testfs/dp_dne':
      File exists

Patch removes the check of the files createmany, statmany
and smalliomany which are part of lustre/tests. With
the existing check test_smallio() is always skipped
if run on PWD != lustre/tests.

Patch removes the comparison with MDSSIZE and adds the
comparison with real mds size. Without this fix the test
works incorrectly if run on existing fs (formatall was not
done during this session: Lustre was created with mds size
differs from MDSSIZE).

Add skip_env() to report about missing mdtest/dbench/IOR/etc.
Without this fix  the tests are skipped silently.

Test-Parameters: trivial mdssizegb=20 testlist=dom-performance
Cray-bug-id: LUS-7349
Signed-off-by: Elena Gryaznova <elena.gryaznova@hpe.com>
Reviewed-by: Vladimir Saveliev <vladimir.saveliev@hpe.com>
Reviewed-by: Alexander Boyko <alexander.boyko@hpe.com>
Change-Id: I5fdf1ad480edb17598cbe427bc550396ebe97808
Reviewed-on: https://review.whamcloud.com/37644
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Mike Pershin <mpershin@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Shaun Tancheff <shaun.tancheff@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>