Fix manila's devstack plugin for using Fedora/CentOS/RHEL distro

Move distro-specified commands into special conditions.
Make changes to related places.

Change-Id: I386e70b1f173909ccd5a31f3207047d54f7bc0d0
Closes-Bug: #1271100
Merge "Fix manila's devstack plugin for using Fedora/CentOS/RHEL distro"