[DBaaS] [FE] Ability to create single psmdb node cluster

Problem: For MongoDB Cluster creation, PMM is not allowing a "Single node." cluster mode, so the minimal mongoCluster is  =3. As for experiments and testing, it's OK to have a non-replicated non-shared cluster. 

User story:  as a DbaaS user (Application Developer) I need to be able to create the smallest mongoDB cluster using DBaaS so that I'm able to play with DBaaS and mongoDB  

UI/UX: 

• Topology "Single Node" available for mongoDB cluster 

•  If "Single node" is selected for any technology  - The user sees a warning about "unsafe configuration, not for production use " (TBD) 

Warning like  :

Acceptance criteria 

• User able to select Single note  Topology for  MongoDB cluster creation 

• User warned about unsafe configuration in case of Single node 

Out of scope:

TBD

Suggested implementation:
TBD

How to test:

• User able to select Single note  Topology for  MongoDB cluster creation 

• User see warning about unsafe configuration in case of Single node

Details:
TBD