Fix functions server port assignment

[apuyou]

- Summary

This PR assigns a port for the functions server in the dev command, sets it in the config and then uses it when starting the functions server. Previously, duplicate logic could result in the functions server starting on a specific port and then the CLI waiting for it on another port. This would prevent startup of the whole dev environment (endless dots).

The port is elected as follow:

• functionsPort option in netlify.toml if provided
• 34567 otherwise

If the elected port is currently in use, a random port is assigned.

This fixes the following issue: #659

- Test plan

The test must be run in a context where the functions server is started. This requires a .functions directory with appropriate contents.

The code was tested with the following configuration:

• no port specified in netlify.toml
• no port specified in netlify.toml but port 34567 is busy
• port 11111 specified in netlify.toml
• port 11111 specified in netlify.toml but port 11111 is busy

- Description for the changelog

Fix function servers port assignment issue preventing netlify dev startup

- A picture of a cute animal (not mandatory but encouraged)

Edit: removed #634 from fixed issues

[RaeesBhatti]

Thanks for the PR @apuyou

[mrlubos]

Nice one @apuyou, thanks!