Allocate mux in master.New()

Callsites no longer allocate a mux.
Master now exposes method to install handlers
which use the master's auth code.  Not used
but forks (openshift) are expected to use these
methods.  These methods will later be a point
for additional plug-in functionality.
Integration tests now use the master-provided
handler which has auth, rather than using the mux,
which didn't.  Fix TestWhoAmI now that /_whoami
sits behind auth.
Merge pull request #2044 from erictune/fix_mux

Allocate mux in master.New()