Because the default Ubuntu Xenial repositories recently added a newer version of Ceph Jewel (10.2.11-0ubuntu0.16.04.1), you will get "unmet dependency" errors when trying to install the downstream Red Hat version:

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root@localhost:~# apt-get install ceph-mon
Reading package lists... Done
Building dependency tree       
Reading state information... Done
Some packages could not be installed. This may mean that you have
requested an impossible situation or if you are using the unstable
distribution that some required packages have not yet been created
or been moved out of Incoming.
The following information may help to resolve the situation:

The following packages have unmet dependencies:
 ceph-mon : Depends: ceph-base (= 10.2.10-37redhat1xenial) but it is not going to be installed
E: Unable to correct problems, you have held broken packages.
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I had set up my Ubuntu 16.04 system with the downstream repos, acccording to the installation guide https://access.redhat.com/documentation/en-us/red_hat_ceph_storage/2/html-single/installation_guide_for_ubuntu/#enabling_the_red_hat_ceph_storage_repositories :

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root@localhost:~# cat /etc/apt/sources.list.d/*
deb https://tserlin@redhat.com:XXXXX@rhcs.download.redhat.com/2-updates/MON xenial main
deb https://tserlin@redhat.com:XXXXX@rhcs.download.redhat.com/2-updates/OSD xenial main
deb https://tserlin@redhat.com:XXXXX@rhcs.download.redhat.com/2-updates/Tools xenial main
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This can be remedied by creating an APT preferences file, like below:
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root@localhost:~# cat /etc/apt/preferences.d/rhcs2
Package: *
Pin: origin "rhcs.download.redhat.com"
Pin-Priority: 1001
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