Fixed a flaky test case in reservation tests.
Fixed a flaky test case in reservation tests.

ReservationTest.MasterFailover could fail when parameterized with
the 'RESOURCE_PROVIDER' capability. After the simulated master
failover, the re-registered framework could receive an offer before
the re-registered agent sends a 'UPDATE_SLAVE' message. On receiving
this message, the master would rescind the offer and send out a new
one. The test expected a single offer to be send. It has been changed
to accept the first offer and ignore subsequent ones.

Also, the test case has been updated to run with paused clock.

Review: https://reviews.apache.org/r/64247/

dforsyth/mesos,Gilbert88/mesos,dforsyth/mesos,dforsyth/mesos,reneploetz/mesos,dforsyth/mesos,asamerh4/mesos,Gilbert88/mesos,reneploetz/mesos,chhsia0/mesos,jpeach/mesos,andschwa/mesos,andschwa/mesos,gsantovena/mesos,abudnik/mesos,jpeach/mesos,gsantovena/mesos,kaysoky/mesos,gsantovena/mesos,andschwa/mesos,kaysoky/mesos,andschwa/mesos,asamerh4/mesos,chhsia0/mesos,jpeach/mesos,reneploetz/mesos,reneploetz/mesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,asamerh4/mesos,abudnik/mesos,gsantovena/mesos,jpeach/mesos,gsantovena/mesos,abudnik/mesos,asamerh4/mesos,Gilbert88/mesos,kaysoky/mesos,Gilbert88/mesos,abudnik/mesos,Gilbert88/mesos,gsantovena/mesos,kaysoky/mesos,kaysoky/mesos,jpeach/mesos,andschwa/mesos,asamerh4/mesos,asamerh4/mesos,andschwa/mesos,jpeach/mesos,asamerh4/mesos,dforsyth/mesos,dforsyth/mesos,gsantovena/mesos,dforsyth/mesos,abudnik/mesos,abudnik/mesos,reneploetz/mesos,chhsia0/mesos,chhsia0/mesos,kaysoky/mesos,chhsia0/mesos,chhsia0/mesos,abudnik/mesos,reneploetz/mesos,Gilbert88/mesos,Gilbert88/mesos,kaysoky/mesos,chhsia0/mesos