Option to remove a "not ready" taint at startup

[vflaux]

Allow spegel to remove a taint at startup.
This allow to create nodes with a taint that prevent pods to be schedule. Spegel then remove the taint when ready.

fix #587

• New code has tests where applicable
• The change has been added to the changelog.
• Documentation has been generated if applicable.
• The unit tests pass.
• Linter does not report any errors.
• All end to end tests pass.

[vflaux]

I'm not yet satisfied with the fact that the removal is done immediately after starting the server & shutdown routines without a real validation. The server could have stopped immediately (listening the socket may have failed) and the taint would still be removed.

Edit:
I moved net.Listen() calls to the "main" goroutine in order to fail before deleting the taint.
We could also add a delay before removing the taint to wait for other startup errors, but I'm not a big fan of arbitrary delays.