Miscellaneous last minute 5.1 fixes

operations/2410-galera-autorebuild -- couple syntax fixes

operations/2800-experimental -- added info about when you can
enable experimental features.  I kept it just a little vague,
given the state of Zabbix.

planning-guide/0010-intro -- made a bullet list of alternate ways
to run MOSt and added the same note as is in the Release Notes.

planning-guide/4000-planning-summary -- added vCenter, vSphere,
NSX, and Sahara to the list

planning-guide/4450-monitoring -- moved Ceilometer before Zabbix,
marked Zabbix as experimental.

preface/preface -- deleted 5.0 and 5.0.1 lines

Release Notes/010 -- experimental features can be enabled on a
running Fuel Master, not before it is installed.

Release Notes/050-known-issues -- added LP1372825, that Zabbix
does not work.

Release Notes/060-obtain-the-product -- added Ron's new verbage
about Mirantis OpenStack Express

terminology/galera -- added two xrefs per Sergii Golovatiuk
terminology/experiemental-features -- a bit of rewording

User Guide -- config-environment/1000-assign-roles -- added note
that the Zabbix role is displayed only if experimental features
have been enabled.

Change-Id: I62c40bbe9dbc2bf6a5cf53bff605908ff18c2427