AppDynamics Tier Name in Credentials

Previously, the tier name provided to the AppDynamics agent was hard-
coded in the component configuration file.  This meant that the only
way to change the tier name was to fork the buildpack; a heavy-weight
exercise.  This change makes the configured tier name, the "default"
choice which can be overridden with a 'tier-name' entry in the service
credentials.

[#75252498][#55][#59]
Merge branch 'cgfrost-75252498'

[resolves #81]
AppDynamics Node Name from Instance Index

Previously, AppDynamics node names were based on the instance's id.
This led to a proliferation of instances in the AppDynamics console
which hindered usability.  This change now uses the instance's index
for its node name.  This will lead to fewer instances in the
AppDynamics console, improving usability.

[#75252406][#55]
Merge branch 'cgfrost-75252406-AppDynamics-nodeName'

[resolves #83]