optionally set mtu for each interface in the env file
dont set mtu until the interfaces have been started.
Added some custom network settings. Moved all BCPC sysctl stuff into own config

Rather than do all the sysctl stuff in bash blocks I use the sysctl fragment and notify sysctl -p to reload them.
Set swappiness back to zero
bump bcpc.mirror.os-dist to icehouse for consistency
moved RGW to port 80/443 on the float
Merge pull request #252 from caiush/mtu_settings

Add the ability to set per interface MTUs
Merge pull request #259 from caiush/rgw_local_ports

moved RGW to port 80/443 on the float
Merge pull request #257 from caiush/sysctl

Use sysctl config fragments to manage bcpc sysctl settings.
explicitly set notify to immediate
migrate networking sysctl's to conf file
allow env file to enable/disable BCPC features
move data bag encryption to enabled/disabled flag
move firewall to enabled/disabled flag
move apt upgrade to enabled/disabled flag
move keepalived checks to enabled/disabled flag
add some comments about what all the bools do
allow network tests to be enabled/disabled
setup zookeeper on headnodes
setup cassandra on headnodes