Create runtime dir in bootstrap

Some hypervisors (KVM) need RUN_GANETI_DIR to exist even at cluster init
time. This patch creates it in InitCluster just before hv parameter
checking. Since the code to make list of directories is already repeated
twice in the code, and this would be the third time, we abstract it into
an utils.EnsureDirs function and we call that one from ganti-noded,
ganeti-masterd and bootstrap.

Reviewed-by: iustinp