Add shallow checkout for azure pipelines

[Trzs]

• bootstrap.py always checks out the full repository
• the azure runner is severely disk limited, so the '.git' directories are deleted again
• kokkos needs git information to be build -> unwieldy work around to delete the '.git' dir in an extra step
• I propose to add --depth=1 to git clone commands in bootstrap.py, so that only the necessary data is downloaded in the first place
• can be reversed, where necessary, with git fetch --unshallow

Note to @nksauter, @bkpoon