qa: test_alloc_hint: set ec ruleset-failure-domain to osd

Create a custom profile with ruleset-failure-domain=osd.  (The default
ruleset-failure-domain=host won't do because this script assumes and
works only if all osds are on the same host.)  While at it, set k and m
explicitly to avoid troubles in the future.

Signed-off-by: Ilya Dryomov <ilya.dryomov@inktank.com>