LU-10070 tests: New test-framework functionality

LU-10070 tests: New test-framework functionality

The self-extending layout tests will make heavy use of
setting OST low & high watermarks to simulate low/out of
space conditions.  To this end, add improved ways of
working with these to the test framework and use them in
sanity 253.

Add a component-count helper in sanity-pfl.

Fix pool_add_targets so it can add only 1 target.

Also move one helper from sanity to test-framework so it
can be used from sanity-pfl.

Cray-bug-id: LUS-2528
Signed-off-by: Patrick Farrell <paf@cray.com>
Change-Id: I4e75c7db07b201ff2c410734d5daa991e74bd5c1
Reviewed-on: https://review.whamcloud.com/33778
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Elena Gryaznova <c17455@cray.com>
Reviewed-by: Patrick Farrell <pfarrell@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>