Skip Postgres backup on Ubuntu 18.04+

[blag]

Mistral is not installed on Ubuntu 18.04+, so backing up Postgres is not required.

As of this writing, StackStorm on Ubuntu 18.04 is still using our "unstable" packages, but once it is released as stable (which should be ST2 3.1), this will be more of an issue.

Original issue: StackStorm/st2#4695

[nmaludy]

This is as easy as just running backups.mongodb_backup instead of backups.full_backup