Restart mysql during o-r-c post-configure

It was not being restarted meaning config changes would not be realized.

Change-Id: I23f6c3a9d0ab8a3c8ed55a1bf4f52c2225988b64
Enable mysqld service in mysql element

Enable mysqld service in mysql element rather than in boot-stack.
This fixes mysql element for systemd distros as previously boot-stack
install.d script tried to enable the service before mysqld had been
installed.

Change-Id: I35f53c0d0ca11e865f522a2b1b366c897fc40da7