Touch up documentation strings for DigitalOcean.
Update playbooks_variables.rst

incorrect/confusing wording in Jinja2 note?
Merge pull request #6347 from dwheaton/patch-1

Update playbooks_variables.rst
Merge pull request #6344 from alimony/patch-1

Touch up documentation strings for DigitalOcean.
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