Increasing max execution time for install (Bug #1318430)

To allow for systems where the installation takes longer than their
phpini max_execution_time setting.

Made the changing of the limit pass thru a function for sanity checking

Change-Id: I0d27d68a377c338c634f2f845829682af149d79f
Signed-off-by: Robert Lyon <robertl@catalyst.net.nz>