Currently a job can be tied to a group of slaves and be executed when such a
slave has idle executors available.

When there are many slaves with multiple capabilities it is sometimes very hard
to maintain this many number of groups to have fine grained control where a job
can be executed.

My suggestion is that if a job can be tied to multiple groups and be executed
only on slaves being part of all these groups that will make things much more
easy to maintain.

Example:
job "X" is tied to groups "linux" "x86_64" "bigmem"
job "Y" is tied to groups "linux" "x86_64"
slave "A" is in "linux" "x86_64"
slave "B" is in "linux" "x86_64" "bigmem"
then job "X" might be executed on slave "B" only
and job "Y" might be executed on both