Infra: ansible update arm perf testbeds

Only isolate as many cores as needed for VPP testing so that the rest
are available for other tasks, such as DPDK compilation.
Configure perf iommu kernel cmdline which works on arm.

Change-Id: I0bf356b34e6a611bd43707b4fd43b7701cd205de
Signed-off-by: Juraj Linkeš <juraj.linkes@pantheon.tech>