[4239] Added drop scripts for both MySQL and Postgresql

[4239] Added drop scripts for both MySQL and Postgresql

Rather than use hard-coded lists or query logic for dropping the
database all MySQL and Posgresql tests use new drop scripts
added to src/bind/admin/scripts.

src/bin/admin/scripts/mysql/dhcpdb_drop.mysql
    - New SQL script to drop a MySQL database

src/bin/admin/scripts/pgsql/dhcpdb_drop.pgsql
    - New SQL script to drop a Postgresql database

src/bin/admin/tests/mysql_tests.sh.in
    - mysql_wipe() - modified to use new drop script

src/bin/admin/tests/pgsql_tests.sh.in
    - pgsql_wipe() modified to use new drop script

src/lib/dhcpsrv/tests/mysql_lease_mgr_unittest.cc
    - TEST(MySqlOpenTest, OpenDatabase) - added show_err=true
    to destroyMySQLSchema() calls

src/lib/dhcpsrv/tests/pgsql_lease_mgr_unittest.cc
    - TEST(PgSqlOpenTest, OpenDatabase) - added show_err=true
    to destroyMySQLSchema() calls

src/lib/dhcpsrv/testutils/mysql_schema.cc
    - destroyMySQLSchema(bool show_err) - modified to use the new
    drop script and accept show_err parameter

src/lib/dhcpsrv/testutils/pgsql_schema.cc
    - destroyPgSQLSchema(bool show_err) - modified to use the new
    drop script and accept show_err parameter