admin: Add setting limit for suspended VMs

Set a limit for the suspended VMs that will be displayed in a user's
summary (default: 10).
admin: Limit number of suspended VMs in summary

When the number of suspended VMs exceeds a pre-defined limit, do not
show more and simply print three dots ("...").

Fix apyrgio/synnefo#263
admin: Add tests for limit on suspended VMs
Merge branch 'feature-admin-limit-suspended' into feature-admin