libcephfs_proxy: identify and copy related keyring files

libcephfs_proxy: identify and copy related keyring files

When a configuration is passed to libcephfs, if keyring information is
not included in that file, libcephfs tries to find a suitable file
containing it. The proxy simulates the same process and does copies of
the identified keyrings. Then the ceph's "keyring" option is updated to
point to the paths of the copied files. This way the the proxy daemon
will be able to load keyring information when it's not present in the
main configuration file.

Signed-off-by: Xavi Hernandez <xhernandez@gmail.com>