Moved command scheduler to use the scheduler library.
Moved command scheduler to use the scheduler library.

This change moves the command scheduler (`mesos-execute`) to use
the scheduler library instead of the driver.

Review: https://reviews.apache.org/r/45008/

neilconway/mesos,zmalik/mesos,zmalik/mesos,yuquanshan/customizedMesos,andschwa/mesos,kaysoky/mesos,craimbert/mesos,kaysoky/mesos,neilconway/mesos,yuquanshan/customizedMesos,asamerh4/mesos,gsantovena/mesos,andschwa/mesos,zmalik/mesos,dforsyth/mesos,Gilbert88/mesos,verizonlabs/mesos,asamerh4/mesos,yuquanshan/customizedMesos,asamerh4/mesos,zmalik/mesos,neilconway/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,craimbert/mesos,craimbert/mesos,neilconway/mesos,asamerh4/mesos,abudnik/mesos,chhsia0/mesos,jpeach/mesos,chhsia0/mesos,dforsyth/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,reneploetz/mesos,yuquanshan/customizedMesos,verizonlabs/mesos,jpeach/mesos,abudnik/mesos,kaysoky/mesos,chhsia0/mesos,Gilbert88/mesos,kaysoky/mesos,craimbert/mesos,andschwa/mesos,verizonlabs/mesos,abudnik/mesos,andschwa/mesos,chhsia0/mesos,zmalik/mesos,Gilbert88/mesos,jpeach/mesos,verizonlabs/mesos,gsantovena/mesos,verizonlabs/mesos,Aman-Jain-14/customizedMesos,reneploetz/mesos,andschwa/mesos,kaysoky/mesos,chhsia0/mesos,dforsyth/mesos,gsantovena/mesos,Aman-Jain-14/customizedMesos,abudnik/mesos,Aman-Jain-14/customizedMesos,kaysoky/mesos,craimbert/mesos,jpeach/mesos,reneploetz/mesos,gsantovena/mesos,gsantovena/mesos,reneploetz/mesos,chhsia0/mesos,Gilbert88/mesos,kaysoky/mesos,dforsyth/mesos,chhsia0/mesos,asamerh4/mesos,yuquanshan/customizedMesos,reneploetz/mesos,abudnik/mesos,Aman-Jain-14/customizedMesos,dforsyth/mesos,jpeach/mesos,zmalik/mesos,zmalik/mesos,gsantovena/mesos,asamerh4/mesos,andschwa/mesos,neilconway/mesos,dforsyth/mesos,verizonlabs/mesos,neilconway/mesos,gsantovena/mesos,verizonlabs/mesos,abudnik/mesos,jpeach/mesos,craimbert/mesos,yuquanshan/customizedMesos,reneploetz/mesos,asamerh4/mesos,Gilbert88/mesos,Aman-Jain-14/customizedMesos,jpeach/mesos,andschwa/mesos,reneploetz/mesos,dforsyth/mesos,neilconway/mesos,craimbert/mesos,yuquanshan/customizedMesos