Relocate SERVICE_TIMEOUT to stackrc

cinder_driver_cert.sh restarts volume services
and needs the SERVICE_TIMEOUT variable set, but
that was being declared in stack.sh.

Rather than create another duplicate variable in
the cert script, just move the SERVICE_TIMEOUT
variable to stackrc so it can be shared like other
common variables.

Change-Id: I650697df015fed8f400101a13b6165ac39626877
Closes-Bug: 1350221
Merge "Relocate SERVICE_TIMEOUT to stackrc"