Used agent environment variables for I/O switchboard.
Used agent environment variables for I/O switchboard.

THe I/O switchboard server process will inheirt most of the agent
environment variables, except for those that are prefixed with MESOS
or LIBPROCESS since we don't rely on those environment variables.

The patch also manually set a few LIBPROCESS environment variables to
make sure the libprocess for the I/O switchboard can be initailzied
properly without depending on external environment.

Review: https://reviews.apache.org/r/54487

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