lxc: add a new lxc.mount.entry keyword

lxc: add a new lxc.mount.entry keyword

The purpose of this new keyword is to save in main config file
all the lines of a provided fstab file.
This will ultimately replace the the lxc.mount keyword
when lxc scripts will use the new keyword.

Warning: I did not validated this patch
in all conditions of provided malformed input string.

Signed-off-by: Michel Normand <michel_mno@laposte.net>
Signed-off-by: Daniel Lezcano <dlezcano@fr.ibm.com>