linuxmce #2721 #2762 #2788 - xenial / mysql-5.7

These tickets affect the main database and asterisk
connectivity on fresh install.

Adjust SQL syntax for creating db users.
Don't pass from the postinst if the db user creation fails,
instead use SQL statements to pass if the user exists.
This stops the postinst passing if there was an error
and to catch the error.

Change the syntax of setting the password of the main
database for xenial / mysql-5.7. Not particularly happy about
doing it this way but this should restore the old
functionality.

Create empty database for pluto_main in postinst because it
doesn't exist yet on a fresh install and this causes a chain
of errors going further through the package install list.