REGTEST: replace LEVEL option by a more human readable one.

This patch replaces LEVEL variable by REGTESTS_TYPES variable which is more
mnemonic and human readable. It is uses as a filter to run the reg tests scripts
where a commented #REGTEST_TYPE may be defined to designate their types.
Running the following command:

    $ REGTESTS_TYPES=slow,default

will start all the reg tests where REGTEST_TYPE is defines as 'slow' or 'default'.
Note that 'default' is also the default value of REGTEST_TYPE when not specified
dedicated to run all the current h*.vtc files. When REGTESTS_TYPES is not specified
there is no filter at all. All the tests are run.

This patches also defines REGTEST_TYPE with 'slow' value for all the s*.vtc files,
'bug' value for al the b*.vtc files, 'broken' value for all the k*.vtc files.
diff --git a/reg-tests/stick-table/b00001.vtc b/reg-tests/stick-table/b00001.vtc
index 3435712..d4fb862 100644
--- a/reg-tests/stick-table/b00001.vtc
+++ b/reg-tests/stick-table/b00001.vtc
@@ -14,6 +14,8 @@
 #
 # This should be backported to 1.8
 
+#REGTEST_TYPE=bug
+
 varnishtest "stick-tables: Test expirations when used with table_*"
 
 # As some macros for haproxy are used in this file, this line is mandatory.