Extract repetitive code into a function

Two functions were introduced for manipulating the packstack answer
file:
* Use a function for modifying a configuration value with a given.
* Use a function to disable unneeded services.
Rename template to contain plugin type

The template is renamed to contain the ml2 plugin type.
The change is required to distinguish from the ovs plugin
which will be introduced due to issues of packstack to
configure ml2 plugin + ovs + vlan.