Made the command executor use the adapter interface.
Made the command executor use the adapter interface.

This change adds support to the command executor to toggle
between the driver based adapter/v1 API based on the
`MESOS_HTTP_COMMAND_EXECUTOR` environment variable
set by the agent.

Review: https://reviews.apache.org/r/47366/

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