helm: Add tiller service account

Initialize tiller service account with cluster-admin role [1].
While at it, use multiarch tiller image from [2], as recommended
in [3], at least until upstream gcr.io repo gets proper multiarch
support.

[1] https://helm.sh/docs/using_helm/#role-based-access-control
[2] https://github.com/jessestuart/tiller-multiarch
[3] https://github.com/helm/helm/issues/3269

Change-Id: I3719b648de0cbec9fc8802c74763c45233a04861
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>