Adds MySQL as an option for integration specs.

To use MySQL, specify DB=mysql in your environment. Any other value will
cause Postgres to be used.

Unquotes double quotes in apply spec string, as MySQL stores them
quoted, or at the adapter renders the string with escaped quotes in the
tasks list. Postgres doesn't show this behavior, so not unescaping broke
our integration tests.

Signed-off-by: Matthew Boedicker <mboedicker@pivotallabs.com>