Dependency to galera on ubuntu 12.04 is missing:
Depends: mariadb-client-5.5 (>= 5.5.29-mariadb1~precise), libdbi-perl, perl (>= 5.6), libaio1 (>= 0.3.93), libc6 (>= 2.15), libpam0g (>= 0.99.7.1), libssl1.0.0 (>=
1.0.0), libstdc++6 (>= 4.6), zlib1g (>= 1:1.2.0), debconf (>= 0.5) | debconf-2.0, psmisc, passwd, lsb-base (>= 3.0-10), libmariadbclient18 (>=
5.5.29-mariadb1~precise)

So installing mariadb-galera-server-5.5 will not be sufficient for a cluster.
On the other hand the debian and rpm packages ships with a dependency to galera.
Which is inconsistent.

Regards
Erkan