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
base: drop $ISO_MOUNT_POINT global variable

clean_mount_points() will now scan /proc/mounts to identify filesystem
to umount. That's why there is no need to keep a trace of the ISO mount
point manually.
base: do not install ifupdown on RPM distro
base: bootstrap() reentrant if DIST=centos

ensure we can run bootstrap() two time in a row with DIST=centos.
this my look a bit hackish, maybe we can do a rpm wrapper to catch
this situation where rpm return 1 because the rpm is already here.