Script tweaks; use standard env var names in multiversion/checkoutMediaWiki

Replace $commonHomeDir / $commonLocalDir vars with MULTIVER_COMMON_HOME
/ MULTIVER_COMMON_APACHE constants, for consistency with deployment scripts,

Calls to updateBitsBranchPointers.php embedded in checkoutMediawiki and
deleteMediawiki.

Change-Id: I3f390c4c3bafa1a349411b9bf52955f8b58212e1