Upload CA cert in the directory on install

This will later allow clients to securely download the CA cert by
performaing mutual auth using LDAP with GSSAPI
Update plugin to upload CA certificate to LDAP

Define post-update plugin to upload public CA certificate to IPA LDAP server.
The plugin includes also update file that creates default container for the
certificate.
Do SSL CA verification and hostname validation.
Use secure method to acquire IPA CA certificate

Major changes ipa-client-install:

* Use GSSAPI connection to LDAP server to download CA cert (now
  the default method)

* Add --ca-cert-file option to load the CA cert from a disk file.
  Validate the file. If this option is used the supplied CA cert
  is considered definitive.

* The insecure HTTP retrieval method is still supported but it must be
  explicitly forced and a warning will be emitted.

* Remain backward compatible with unattended case (except for aberrant
  condition when preexisting /etc/ipa/ca.crt differs from securely
  obtained CA cert, see below)

* If /etc/ipa/ca.crt CA cert preexists the validate it matches the
  securely acquired CA cert, if not:

  - If --unattended and not --force abort with error

  - If interactive query user to accept new CA cert, if not abort

  In either case warn user.

* If interactive and LDAP retrieval fails prompt user if they want to
  proceed with insecure HTTP method

* If not interactive and LDAP retrieval fails abort unless --force

* Backup preexisting /etc/ipa/ca.crt in FileStore prior to execution,
  if ipa-client-install fails it will be restored.

Other changes:

* Add new exception class CertificateInvalidError

* Add utility convert_ldap_error() to ipalib.ipautil

* Replace all hardcoded instances of /etc/ipa/ca.crt in
  ipa-client-install with CACERT constant (matches existing practice
  elsewhere).

* ipadiscovery no longer retrieves CA cert via HTTP.

* Handle LDAP minssf failures during discovery, treat failure to check
  ldap server as a warninbg in absebce of a provided CA certificate via
  --ca-cert-file or though existing /etc/ipa/ca.crt file.

Signed-off-by: Simo Sorce <simo@redhat.com>
Signed-off-by: Rob Crittenden <rcritten@redhat.com>
Don't initialize NSS if we don't have to, clean up unused cert refs

Check to see if NSS is initialized before trying to do so again.

If we are temporarily creating a certificate be sure to delete it in order
to remove references to it and avoid NSS shutdown issues.

In the certificate load validator shut down NSS if we end up initializing
it. I'm not entirely sure why but this prevents a later shutdown issue
if we are passed the --ca-cert-file option.
Update anonymous access ACI to protect secret attributes.

Update anonymous access ACI so that no users besides Trust Admins
users can read AD Trust key attributes (ipaNTTrustAuthOutgoing,
ipaNTTrustAuthIncoming). The change is applied both for updated
IPA servers and new installations.