add lti variable substitution for $Canvas.api.baseUrl

test plan
 - install a tool with the $Canvas.api.baseUrl custom param
 - it should substitute with scheme, host, and port

fixes PLAT-668

Change-Id: I7ca633fd3e44dcf632aefb59491866554636226f
Reviewed-on: https://gerrit.instructure.com/42119
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Brad Humphrey <brad@instructure.com>