ec2_ami: Account for AWS's "eventual consistency" with AMI creation.

Calling `ec2.get_image` right after `ec2.create_image` may raise error
"InvalidAMIID.NotFound". This has happend roughly 1 time out of 10 for me.

Other people has bitten by this too:
- https://github.com/Netflix/aminator/commit/5707f100a07d3432027d1e7a3305567a2a7d2574
- http://stackoverflow.com/a/14794952
- https://bitbucket.org/utoolity/bamboo-aws-plugin/pull-request/22/baws-116-fix-ec2-image-task-failing-with/diff
Merge pull request #4370 from hostage-nl/devel

replace dashes to underscores in rc.conf keys (based on freebsd rc.d filenames)
Allow specifying an alternate ec2.ini path via EC2_INI_PATH environment variable.
Allow for updating host vars in add_host
Merge pull request #4376 from cchurch/devel

Support for alternate ec2.ini path via EC2_INI_PATH environment variable
Merge pull request #4364 from stoned/pre-1.6-passlib

Fix compatibility issue with passlib version < 1.6 (as encountered on eg...
add more indexes, AWX blurb.
Add lots of anchor tags.
More exciting anchor tags, document more loop features!
More anchors!
Misc docs work.
Remove duplicate section, fix a link.
Fix some docs formatting.
Add see also sections to pages that do not have them.
Remove duplicate link to old examples dir.
Does "fix malformed hyperlink target" sound any less Star-Trekky than "realign the main deflector array?".
Fix some missing hyperlinks to ansible-examples on github.
Add toctrees to pages in the docs that did not have them.
Various docs cleanup.
Sphinx formatting.