Remove collectd_hostname attribute

`collectd_hostname` was being used to populate the Collectd
configuration file's `Hostname` setting. If omitted, Collectd uses the
`gethostname(2)` system call to automatically set `Hostname`. This
behavior is much more desirable, especially in dynamic environments like
EC2.
0.1.1
Merge branch 'release/0.1.1' into develop