Make the listen IP address configurable

When running cAdvisor in a container the listen IP is not really important.  Since cAdvisor is still just a simple go program you can easily run it outside of a container in the host OS.  When running cAdvisor in the host OS it is nice to be able to specify which IP cAdvisor listens on, for example 127.0.0.1.
nit: while we're here :) can we change to %d?