fixes for current issues

[mvasilenko]

Please review change that fix current issues:

• add ppa for redis, then update package cache
• nginx role fixes
• ansible 2.0 compatibility
• redis-py specific version requirement (current version 2.10.6 will not work)
• django template updates
• install ansible inside VM
• make sentry role more idempotent, by checking sentry superuser existence before creating
• update ubuntu version