Take advantage of a new feature in charmhelpers to ignore missing keys
when running sysctl. This addresses the issue where we want to include
conntrack settings for nova-compute with a kvm driver, but don't want
to include those settings for lxd. And it addresses it in a generic
way to handle similar situations going forward.
Change-Id: I29c36c11345b8876c4c39651c124a64860dd35db
Closes-Bug: 1820635
1304a492b1