scripts: scripts-library.sh: Hide 'which' command output
scripts: scripts-library.sh: Hide 'which' command output

If systemd-resolve is not available, then we get the following error
in the output

which: no systemd-resolve in (/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/sbin:/usr/bin:/sbin:/bin)

We only care about the 'which' exit code so lets just hide
stdout/stderr.

Change-Id: Ia6597e68d8237f6d5ab2acb86116dbaa31465e12

cfarquhar/openstack-ansible,bunchc/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,cfarquhar/openstack-ansible,openstack/openstack-ansible,stackforge/os-ansible-deployment,bunchc/openstack-ansible,yanyao/openstack-ansible,yanyao/openstack-ansible