upgrade-from : clean mountpoints before clear trap

As we do clear the trap, we have to clean mountpoint manually unless
/proc & /dev remains mounted.
base.install: install RHEL base from ISO

Just set ISO_PATH to the full path of your rhel ISO instead of
settings the REPOSITORY variable.
local: can only be used in a function

we use "local" to flag variable as local. The key word is only
available within a function.

this is a partial reverse of 48b320be3dbba68811c
common: simplify clean_mount_points()

This patch simplify the clean_mount_points() behavior:
- no need anymore to list the mounted filesystem
- use only one while loop and so drop a level of complexity
base: clean_mount_points() already calls unmount

There is no need to call umount during the script execution.
clean_mount_points() should be called at the end of the script instead.
cleanup(): drop mostly duplicated function

A cleanup() function is already defined in common which do the same
things:
 - catch error by reading $ret
 - umount related filesystem, including ISO_MOUNT_POINT
 - generated .edeploy archive