place default keystore to install space

[mikaelarguedas]

This PR implements the keystore location in install space, based on discussion from #101 (comment)

The original implementation aimed at creating the default keystore in the sros2_cmake workspace install space under a ros2_security directory.

https://github.com/ross-desmond/sros2/blob/51fd900a1665bd6a4d760aa501488401f32946f3/sros2_cmake/sros2_cmakeConfig.cmake.in#L3-L5

In the case of installing from packages, this logic results in creating the keystore at /opt/ros/crystal/share/sros2_cmake/cmake/../../../ros2_security (which requires root access).

Changes from original implementation:

• keystore is created in the current worskpace installation location
  • this requires the same permissions as the other install targets
  • is robust to the installation layout of sros2_cmake's workspace (aka results in the same directory whether the underlay was built in isolated install or merged install layout)
• the keystore folder keys is renamed keystore

@ross-desmond
Does this match the expected behavior?
What files do we expect to be placed in the ros2_security folders other than the keystore? (user policy files maybe?). If none maybe 1 layer of subdirectory would be enough

[mikaelarguedas]

@ross-desmond @mjcarroll gentle 🛎️

[mjcarroll]

LGTM, @ross-desmond can you comment on Mikael's questions above?

[ross-desmond]

@mikaelarguedas this does match the expected behaviour. I believe the comarmor xml policy files are more like a source, as they get compiled into DDS policy files and signed, and are therefore not included in the keystore. The keystore should be able to be generated from scratch to ensure deterministic compilation of security settings. What are your thoughts on that?

[mikaelarguedas]

I believe the comarmor xml policy files are more like a source, as they get compiled into DDS policy files and signed

Yeah I agree with that. While I would expect most people to provide their policies via a package, they should be able to just put them in the install/ros2_security, delete install/ros2_security/keystore and regenerate. So keeping two directory levels for that use case sounds good to support that use case 👍

[mikaelarguedas]

@mjcarroll I think we're on the same page here and this is ready to go. (thanks @ross-desmond for the comments 🙇‍♂️ )