Per the referenced bug, $NOVA_CPU_CONF was previously being initialized
by copying $NOVA_CONF, thereby trashing any values already configured in
$NOVA_CPU_CONF.
With this commit, we merge the values from $NOVA_CPU_CONF in after the
copy.
Note that this makes use of the merge_config_file function, which is
defined in inc/meta-config, which wasn't being sourced from every code
path that hit start_nova_compute; so this commit also moves that import
from stack.sh to functions (next to the other imports from inc/, which
makes sense anyway).
Change-Id: Id3e2baa2221e13f512f8dcf1248e1e15b6a7597f
Closes-Bug: #1802143
2468ceaa72