This was already discussed in #88 and was supposedly fixed, but did not
work for me. Now aliases are possible with correct regexp format (no
colon). Alias for postmaster is omitted in that case (incorrect regexp
format with colon). Defaults are unchanged.

Makes it possible to specify alias database type independent of
postfix_default_database_type. Use the new variable for the alias_maps
and alias_database configuration options. Default is the value of
"postfix_default_database_type".

This is the minimum invasive change to support regexp expressions in
aliases without changing other functionality of the role, especially
default database types for other configuration mappings.