packaging: allow importing ISO domain on setup, exports.d support

moved the private function engine_validators._isPathInExportFs
to common_utils.isPathInExportFs and made it public.
isPathInExportFs previously redefined the built-in file,
I've fixed it.

engine_validators.validateMountPoint now accept an existing ISO
domain as a valid path even if it's already exported allowing the
user to re-import it.

engine-setup._configNfsShare need to add the ISO domain path to
/etc/exports only if it is not already there.
If /etc/exports.d exists, then
/etc/exports.d/ovirt-engine-iso-domain.exports
will be used instead of /etc/exports and in this case
existing configuration will be migrated to
/etc/exports.d/ovirt-engine-iso-domain.exports as well.

engine-setup._configNfsShare generate a new uuid and create a new
domain only if it doesn't already exists in the supplied domain path.

nfsutils.cleanNfsExports refactored taking advantage of the new
private method _updateNfsExports.

engine-cleanup: add support for /etc/exports.d

Change-Id: I3a4343f8374e6439b0ee0a14fff233a0d4a0d5a3
Bug-Url: https://bugzilla.redhat.com/904149
Bug-Url: https://bugzilla.redhat.com/907899
Signed-off-by: Sandro Bonazzola <sbonazzo@redhat.com>