Add the posibility to start the database
without a pre-existing database dump by
adding an -e switch to
sider snapshot add.

This will start the chose database image with an empty
mapped folder.

Test plan:
* Create a snapshot with the sider snapshot add -e
  <redis> <name> for each existing engine.
* Use the database by starting a db on the created
  snapshot (sider db start <name> <name>).