GUACAMOLE-32: extended variable support for linked databases

[vchrisb]

• adding support for leveraging MYSQL_ENV_MYSQL_USER, MYSQL_ENV_MYSQL_PASSWORD, MYSQL_ENV_MYSQL_DATABASE and MYSQL_ENV_MYSQL_ROOT_PASSWORD for linked
• removed deprecated environmental variables MYSQL_NAME, MYSQL_PORT_3306_TCP_ADDR and MYSQL_PORT_3306_TCP_PORT
• adding support for leveraging POSTGRES_ENV_POSTGRES_USER, $POSTGRES_ENV_POSTGRES_PASSWORD and POSTGRES_ENV_POSTGRES_DB for linked
• removed deprecated environmental variables POSTGRES_NAME, POSTGRES_PORT_5432_TCP_ADDR and POSTGRES_PORT_5432_TCP_PORT

[mike-jumper]

1. Will removing support for the old form cause things to break?
2. Why the variable initialization starting with colons? I'm not familiar with this syntax.
3. Please follow the code style of the rest of the script - 4 space indent, no tabs.
4. Why all the new defaults? If the goal is to leverage additional variables, changing the behavior of default values seems out of scope.

[vchrisb]

1. I'm not aware of any drawbacks. It should be 100% backward compatible. That's the reason why I haven't renamed MYSQL_PORT, though it is preset with something like tcp://172.17.0.5:5432. See name_PORT in Link environment variables reference
2. Please have a look at Example 10-7 in Parameter Substitution
3. I can rewrite the variable assignment using if statements.
4. new defaults are for MYSQL_HOSTNAME and MYSQL_PORT because the old are deprecated. Defaults for MYSQL_USER is, as commented above, discussable

[necouchman]

Pinging on this one, trying to clean up some old PRs. @mike-jumper: any other comments for Chris, here, after his last post?