[5.x] Allow specifying permissions for Glide cache

[duncanmcclean]

This pull request allows for overriding the permissions used to write to the Glide image cache.

Closes #8199.

[jasonvarga]

Adding a config for the 'cache' disk is confusing here. It only applies when you have 'cache' => false. If you enable the "cached" version, it doesn't apply. Whatever config we use here would probably conflict with what's happening in #10204. I'll just make a note in there to handle this issue.