Use dependencies to wait for P/PE/CE nodes before starting the server

[hellt]

@tiago-amado @mairp @alfilipe

this is a hotfix to make this lab compatible with clab 0.52+

I shared the details with @alfilipe, the culprit is that the web app doesn't handle network unreachable nodes when trying to fetch the port status with gnmi.

In this PR I used the stages dependency to ensure that the client7 node executes its setup in the configure stage after all routers are rendered healthy. This makes the ports reachable by the time the flask app starts.

[mairp]

hi @hellt excellence!, actually this is a really good example of how to use stages on clab, please let us know when this is ready.

[hellt]

this is ready, but it would be best if @alfilipe fixes the flask app not to error on unavailable connections

so maybe let's see if he can come up with a solution in a couple of days and if not, we can merge this and say that clab 0.52.0 is the min required version

[hellt]

superseded by #38