Fixing *_plugins configuration names and values

Based on reading of Developing Plugins and the example ansible.cfg file, it seems the sections changed here were created in error by a copy and paste.
Merge pull request #5770 from miksago/patch-1

Fixing *_plugins configuration names and values
Add the start of a coding guidelines document.
Update some things in coding guidelines.
Add note about tests, etc.
Missing colon.
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