Model changes for plots to support multiple y axes

[shefalijoshi]

Is your feature request related to a problem? Please describe.
Based on the design here: https://trunk.arc.nasa.gov/confluence/display/VIPERMS/Multiple+Y-Axes+for+plots
Model changes to the plot configuration to allow more than 1 y axis.

Describe the solution you'd like
Allow a configurable number of Y axis to be instantiated and saved on the plot configuration

Describe alternatives you've considered
N/A

Additional context
This is step zero of the Multiple Y axes implementation for Open MCT plots.