Replace /var/lib/mysql path in the upstart script

The Upstart script for MySQL checks /var/lib/mysql on start, but with
the move to /mnt/state, the directory does not exist, so it fails to
start. Replace the path in the install.d script for the element.

Closes-Bug: 1270017
Change-Id: I1c51c559b585bf561d9e5e0fbcf1569794b9419e
Merge "Replace /var/lib/mysql path in the upstart script"