packages: Adding new functions

This patch add three new methods :
- is_package_installed : return if the requested package is already
  installed
- rebuild_db : rebuild the package database from inside the chroot
- rebuild_db_with_local: rebuild the package database from the host

The two rebuild_db calls are needed with using RPM based distro.
When performing updates on a target, the rpm format could be different
from outside & inside the chroot.
base.install: Calling rebuild_db_*

Let's call rebuild_db insted of manual rpm --rebuilddb call.

One rebuild_db_with_local is mandatory as the rpm format inside the
chroot is not matching the host one.
base.install: Adding more deps for centos/redhat

Thoses are required to get the pxe OS working properly
base.install: Allow multiple passes on install hp

When updating install stuff, we need a check to avoid rpm trying to
install it several time unless it fails.
pxe & health: Reduce initrd size for RPM distro

When building pxe & health targets, let's be sure that we don't embed
useless stuff that makes the initrd to big to be loaded properly