Core: set hostname

Summary:
We always and for all OS set hostname to /etc/hostname.

When this file is modified, we run hostname, then
other OS specific commands.

Fixes T794.

Test Plan:
Tested on Eglide with the following:

• remove /etc/hostname, apply Salt state, read /etc/hostname
• apply Salt state when file won't be modified, see commands don't run

Reviewers: Sandlayth

Maniphest Tasks: T794

Differential Revision: http://devcentral.nasqueron.org/D404