Install Guide: Fix plural

Trivial grammar fix: is -> are

Change-Id: Icd622ca32eb1dc36db8bda073084a3ab194ebba3
backport: havana
Closes-Bug: #1244743
(cherry picked from commit d6205438bb5d8fc8d8f0112c805d77bf57767922)
Install Guide: Add openSUSE and Fedora instruction to Neutron

This updates openSUSE/SLES instructions and also adds in some places
missing Fedora/RHEL/CentOS instructions.

Change-Id: I3b13544744440d34df1e3d3b0f2e8ce97cc146f9
backport: havana
(cherry picked from commit 68c604146687951eee528215cdb40d8ce83a41d2)
Merge "Install Guide: Fix plural" into stable/havana
Merge "Install Guide: Add openSUSE and Fedora instruction to Neutron" into stable/havana