Add nginx roles to plays and add new plays.

Added new plays for splunkforwarder and datadog and added
nginx to the xqueue, xserver, and edxapp plays.
Update to remove preview and add nginx to ora anddiscern.
Merge pull request #624 from edx/feanil/update_plays

Add nginx roles to plays and add new plays.
Remove non-existent 'splunk' group when creating user

When 'splunk' group doesn’t exist yet, ansible fails when creating the
user. Leaving it empty will create the group anyway.
adding mongo updates to the abbey script
pep8
indent
blank line
changing primary key to the ami-id
Set default sites based on a new variable.
Merge pull request #626 from edx/feanil/update_nginx

CMS should not be default.
Update edx-west plays with new nginx variables.
Set a default value for default_site.
Merge pull request #629 from edx/feanil/fix_nginx_defaults

Set a default value for default_site.
Added mirrored queues for clusters and VHOST support
Merge pull request #628 from edx/feanil/update_edx_west_nginx

Update edx-west plays with new nginx variables.
adding datadog and splunk to abbey
gather_facts set to false to speed up run
adding datadog and splunk to abbey
Merge pull request #625 from tusbar/splunk-no-group

Remove non-existent 'splunk' group when creating user