webadmin,userportal: Consolidate gwt.userAgent property

1, gwt.userAgent default value is now Firefox (only)

2, all-user-agents Maven profile can now be used to
   build oVirt GWT UI for all supported browsers
   (no need to enumerate all browsers anymore)

3, Opera excluded from all-user-agents profile

4, in Makefile, DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS
   now defaults to empty value - gwt.userAgent will
   fall back to Firefox (only)

5, in Makefile, BUILD_ALL_USER_AGENTS option can now
   be used to trigger all-user-agents Maven profile

Example 1: build for Firefox (only)

  $ make clean install-dev PREFIX=$HOME/ovirt-engine

Example 2: build for Firefox and Chrome

  $ make clean install-dev PREFIX=$HOME/ovirt-engine \
    DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS="-Dgwt.userAgent=gecko1_8,safari"

Example 3: build for all supported browsers

  $ make clean install-dev PREFIX=$HOME/ovirt-engine \
    BUILD_ALL_USER_AGENTS=1

6, updated ovirt-engine.spec.in by adding variable
   ovirt_build_all_user_agents which defaults to 1

7, updated README.developer

Note that DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS takes
precedence over BUILD_ALL_USER_AGENTS - in example
below, oVirt GWT UI will be built for Firefox and
Chrome:

  $ make clean install-dev PREFIX=$HOME/ovirt-engine \
    DEV_EXTRA_BUILD_FLAGS_GWT_DEFAULTS="-Dgwt.userAgent=gecko1_8,safari" \
    BUILD_ALL_USER_AGENTS=1

Change-Id: I19f5009bb9a73bc554d7ffbd39075130c615c34f
Signed-off-by: Vojtech Szocs <vszocs@redhat.com>