Enhance the user experience with labels and constraints

[deviantony]

Is your feature request related to a problem? Please describe.
Portainer already supports labels on Services & Containers and contraints on Services, but the UI could be improved.

Describe the solution you'd like

• For labels we could list all the available labels from the nodes as well as the available values for these so that the user can choose from existing pair of labels/values.
• For constraints we could list the possible constraints docker allows in a dropdown which the user can select from, so then the user just has to define the value.

This eliminates the need to remember docker label & constraint syntax.