Add support for ENV of the form: ENV name=value ...

still supports the old form: ENV name value

Also, fixed an issue with the parser where it would ignore lines
at the end of the Dockerfile that ended with \

Closes #2333

Signed-off-by: Doug Davis <dug@us.ibm.com>

builder/parser/testfiles/env/Dockerfile

@@ -0,0 +1,15 @@
+FROM ubuntu
+ENV name value
+ENV name=value
+ENV name=value name2=value2
+ENV name="value value1"
+ENV name=value\ value2
+ENV name="value'quote space'value2"
+ENV name='value"double quote"value2'
+ENV name=value\ value2 name2=value2\ value3
+ENV name=value \
+    name1=value1 \
+    name2="value2a \
+           value2b" \
+    name3="value3a\n\"value3b\"" \
+	name4="value4a\\nvalue4b" \