security: use a ramdisk for the temporal keyrings in keymanager

we're writing private keys to the temporary gnupg keyrings in keymanager.

If we have fuse and pyfuse, we can create a ramdisk and pass it to keymanager as an optional argument. I have some working prototypes based on pyfuse examples.

My idea is to have bitmask-core start a RamDiskService, if the requirements are met, and request a new ramdisk for each keymanager instance.

(from redmine: created on 2016-05-17, relates #8106 (closed), relates #8169)