ktest: Create variables for the ktest config files

I found that I constantly reuse information for each test case.
It would be nice to just define a variable to reuse.

For example I may have:

TEST_START
[...]
TEST = ssh root@mybox /path/to/my/script

TEST_START
[...]
TEST = ssh root@mybox /path/to/my/script

[etc]

The issue is, I may wont to change that script or one of the other
fields. Then I need to update each line individually.

With the addition of config variables (variables only used during parsing
the config) we can simplify the config files. These variables can
also be defined multiple times and each time the new value will
overwrite the old value.

The convention to use a config variable over a ktest option is to use :=
instead of =.

Now we could do:

USER := root
TARGET := mybox
TEST_SCRIPT := /path/to/my/script
TEST_CASE := ${USER}@${TARGET} ${TEST_SCRIPT}

TEST_START
[...]
TEST = ${TEST_CASE}

TEST_START
[...]
TEST = ${TEST_CASE}

[etc]

Now we just need to update the variables at the top.

Signed-off-by: Steven Rostedt <rostedt@goodmis.org>