Execute mongoBackups on Clusters, not Services

The problem: UI now allows to select  Service for mongo backup.

But users need the option to select Cluster for mongo, as mongoDB Clusters is the main element for backup.

 Currently, it causes confusion because we take cluster backup for mongo, not a specific instance. 

Proposed solution:

Cluster -  full ist of all clusters and  "N/A as  default option ":
Example: 

• <N/A> or  EMPTY  Cluster name 
• <Cluster 1 > (mongoDB)
• <Cluster 2> (mongoDB)
• <Cluster 3> (MySQL)

Service - list of  Services in the selected cluster . in case of N/A  - all Services without cluster .

Example: 

MongoDB Cluster 1  selected :  

Services: 

• Auto (auto select node) - [SELECTED]
• mongo1-1 <Cluster 1 >
• mongo1-2 <Cluster 1 >
• mongo1-3 <Cluster 1 >

Example2: 

• Service 1 (Cluster 3) <MySQL>
• Service 2 (Cluster 3) <MySQL>

Additional notes: 

• Services list - not active for MongoDB  Clusters 
• Cluster detected by label 'cluster' from  PMM inventory