Check pool existence in master init job

[l0kix2]

 /usr/bin/yt create scheduler_pool --attributes '{name=research; pool_tree=default}' --ignore-existing

from here

ytsaurus-k8s-operator/pkg/components/master.go

Line 270 in 2dcaffb

"/usr/bin/yt create scheduler_pool --attributes '{name=research; pool_tree=default}' --ignore-existing",

fails if pool exists

    Failed to attach child "research" to scheduler pool tree "default"
        Pool tree "default" already contains pool with name "research"

--ignore-existing doesn't work for pool, need to be checked with yt exists

Have to be said that it is not really popular use case to re-create ytsaurus spec for the existing cluster