remove /proc/sysrq-trigger mount, default host namespaces true

[philgebhardt]

Background

• Gremlin currently allows a host shutdown in two different ways: using a /proc/sysrq-trigger, and issuing a shutdown from the host's PID namespace. Only the latter is documented, and the former performs a non-graceful shutdown (which isn't documented in Gremlin).
• At the same time, hostPID=true has become a requirement for all container drivers except for the legacy docker driver. It makes sense to flip this to true to minimize issues at install time resulting from incorrect arguments.
  • hostNetwork is in the same situation, though unrelated to shutdown attacks.

Change

• Remove mount of /proc/sysrq-trigger which disables this functionality in Gremlin
• default hostPID=true to enable host attacks without sysrq
• while we're at it, default hostNetwork=true as well, since it is required for reliability management features.