Packaging up a series of CentOS rpms and others, including MariaDB-server 10.0.29 as a reduced mini CentOS 7 "distro." along with all other needed rpms for a functional system may cause anaconda to install MariaDB before "shadow-utils". "shadow-utils" is package which provides commands like "useradd" which is used by the MariaDB-server post-install scriptlet to create the "mysql" user. The scriptlet intentionally, silently ignores failures (in case the user already exists) but this masks a failure if the "useradd" command isn't present on the system yet. As a result of this unpredictable package ordering by anaconda/yum, MariaDB may end up installed, with no "mysql" user on the system.
Add a "Requires:" line to the .spec file for the rpm, to make MariaDB-server explicitly dependent on packages which provide system commands like "useradd" employed by the install scriptlet. This will ensure yum will order the packages correctly.