Spiff up the 'Developing Plugins' page

* Add more cross-documentation references
* Fix some broken links
* Add links to github/source code references
Remove unused variable from authorized_key
Allow keystring passed to authorized_key to contain blank lines and comments
Added ANSIBLE_SSH_PIPELINING option to enable/disable pipelining support

Pipelining will be disabled by default, since it requires users remove
the 'requiretty' option from the servers sudoers configuration.
Fixes #5631 Return a unique list of hosts when joining groups
Add support for Scientific Linux for hostname module

hostname module was lacking support for Scientific Linux, this commit adds it.
Fixes #5656 quote values with multiple columns before calling
the sysctl command. Calling sysctl should also not be true by default
Merge pull request #5655 from scottgilbert/devel

Allow keystring passed to authorized_key to contain blank lines and comments
Merge pull request #5658 from rishid/devel

Add support for Scientific Linux for hostname module
Removing colon from example failure message

The example failure message was given as - fail: msg="Bailing out: this play requires 'bar'"

The colon in the example causes a YAML compile error, so I am replacing it with a '.'
Merge pull request #5665 from Taytay/devel

Colon in example failure message causes a YAML compile error
Fixes #5661 Handle null values for sysctl parameters
Make synchronize module work better with local transport, fix #5668
Merge pull request #5586 from tbielawa/spiff_up_plugins

Spiff up the 'Developing Plugins' page
fix str format error due to missing '%' in sysctl module
Merge pull request #5672 from simonz05/patch-1

fix str format error due to missing '%' in sysctl module
Merge pull request #5669 from mscherer/fix_synchronize_local

Make synchronize module work better with local transport, fix #5668
update default EC2 exclude regions -- this should be the best configuration for most users, but some may still want to tweak.
Update changelog
Merge upstream changes