Improve release flow

[asmodehn]

We currently test all ros version on every release...
We should find a good way to do like for third-party releases :

1. pure python tests from main branch
2. latest ros distro tests from main branch (if pyros source usecase is in ROS)
3. matching ros distro tests from ros distro branches (if we have some because we support running from source in ROS, for dev for example...)
4. matching ros release from release repo

The goal here is to speed up release while still testing everything needed.

Note that we first need to implement the same system as pyros-dev/pyzmp#9 to make this easier.

[asmodehn]

Note we also need to make ROS dependencies optional (at least for testing...) but in a meaningful way...
So that the pure python test works without ROS dependencies, but still does meaningful tests.