Consider safe_remove for reused output names

[pared]

After merging #1789 use case:

dvc run -o out 'echo 1 >> out'
dvc run -o out 'echo 2 >> out'

described by @efiop in #1786 will behave with respect to recently introduced (#1759) persist flag.
In case of -o new run command will silently remove out before executing new command.
We should consider introducing mechanism using safe remove here, as it might result in unaware data removal.

[efiop]

Closing in favor of #2027