Add ability to set dbms for raw sql changes.

Currently, you can only specify database names at the changeset level.
This commit extends that functionality for individual changes.
Useful for creating more compact change logs.

I'm not fully comfortable with the practive of injecting variables into a changelog from java.
If that is possible, I can move my integration test to common.tests.changelog and pass in the current
database short name so that the testing can be used for all databases.

Currently only testing using H2 but since this feature is just liquibase based it should work fine with
all databases.