Respect region_name in nova_compute

Signed-off-by: Oleg Bulatov <oleg@bulatov.me>
Allow raw and script to support su
Document the `ec2` group.

This group is useful when using multiple inventory sources.
Ensure params were set in ec2_vpc before accessing them

Fixes #6763
Give more information when a task is empty

I made a typo in a playbook and was great by:

    ERROR: expecting dict; got: None

The issue was a single - on the last line of a playbook.
With the name of the file, I was able to see right away where the
error was.
Merge pull request #7126 from mscherer/better_error_message_empty_task

Give more information when a task is empty
update documentation to include missing option

'list' was added more recently, it was omitted from the documentation.
Possible docs error: 'path' instead of 'src'

I came to this example today (line 50), and when I tried it, I got an error:

`msg: src and dest are required for creating links`

So, I changed **path** to **src** and it worked fine. Looks like the example is wrong, since the docs say "dest" is already an alias for "path", in which case we would have 2 "path" definitions.
Merge pull request #7129 from erikaheidi/patch-1

Possible docs error: 'path' instead of 'src'
Merge pull request #7128 from stansonhealth/improve_tag_docu

update ec2_tags documentation to include missing option
Merge branch 'raw-script-su-support' of https://github.com/sivel/ansible into sivel-raw-script-su-support
Merge pull request #6985 from jimi-c/issue_6763_ec2_vpc_params

Ensure params were set in ec2_vpc before accessing them
Merge pull request #6906 from idupree/patch-1

Document the `ec2` group.
Merge pull request #6599 from dmage/nova_region_name

use region_name in nova_compute