Currently, instances will not boot as nova.conf is configured with
`reserved_host_memory_mb: 2048` while the compute VMs themselves only
have 2048MB of RAM assigned.
This commit bumps the compute VMs to have 8192MB of RAM so that
instances can be successfully booted.
NOTE: Prior to all the recent refactors that have taken place, we were
giving the compute VMs 9216MB of RAM. I'm not entirely sure why
this number was chosen but can happily change 8192MB to 9216MB to
match what was being used before.
Change-Id: Ie36fcb4fb3f3b5bce27f2851fba0afc6e0d1966a