Add section about required database privileges

[winem]

This PR adds a section to the documentation to tackle the following error messages you will experience if you did the icinga2 database setup manually and not via the database configuration wizard that grants ALL privileges to the Icinga2 user.

# missing CREATE privilege
mysql -h 127.0.0.1 -p -u icinga2 icinga2 < schema/slaperiods.sql
ERROR 1142 (42000) at line 2: CREATE command denied to user 'icinga2'@'127.0.0.1' for table 'icinga_sla_periods'

# missing ALTER ROUTINE privilege
mysql -h 127.0.0.1 -p -u icinga2 icinga2 < schema/get_sla_ok_percent.sql 
ERROR 1370 (42000) at line 1: alter routine command denied to user 'icinga2'@'127.0.0.1' for routine 'icinga2.idoreports_get_sla_ok_percent'

# missing CREATE ROUTINE privilege 
# be careful, the error message is pretty ambiguous!
mysql -h 127.0.0.1 -p -u icinga2 icinga2 < schema/get_sla_ok_percent.sql 
ERROR 1044 (42000) at line 5: Access denied for user 'icinga2'@'127.0.0.1' to database 'icinga2'

This was tested on Ubuntu 18.04.2 with Icinga r2.10.5-1.

[winem]

Hi @lippserd,

I addressed all your comments and suggestions and pushed all the changes in a squashed commit.

[lippserd]

Thanks for the changes 🎉