Detect remote_user change in accelerate daemon and allow a restart

Fixes #5812
Revert "Merge branch 'slow_ssh' of git://github.com/lberruti/ansible into lberruti-slow_ssh"

This reverts commit fe091ec00d2c646cffbc2e916e89c04febb24426, reversing
changes made to aed2194b1d249c1ebf19bec0cf0c2e6a599f6156.
CHANGELOG update for the new accelerate remote_user feature
Update copy

Update copy documentation to reference synchronize module
Micro-optimization of inventory.expand_hosts.detect_range
Micro-optimization: replace s.find(x)!=-1 with x in s

timeit shows a speedup of ~3x on Python 2.7.5 x86_64.
It also makes the code a bit shorter.
hostname module: don't filter hostnamectl with awk

There's no need to filter hostnamectl's output with awk because its man
page says:

    hostnamectl [OPTIONS...] {COMMAND}

       --static, --transient, --pretty
	   If status is used (or no explicit command is given) and one
	   of those fields is given, hostnamectl will print out just
           this selected hostname.

E.g. hostnamectl --static status => ansible.example.org